Tara Seal Research

A preliminary study of the diet of harbour seals

in Carlingford Lough during the late summer moulting season

Report to the Loughs Agency

December 2012

S.C. Wilson1, M.B. Santos2,3, G.J. Pierce2 and D. Clarke4 1Tara Seal Research, 14 Bridge Street, Killyleagh, Co. Down BT30 9QN.

Email: suewilson@sealresearch.org 2Oceanlab, School of Biological Sciences, University of Aberdeen, Main Street, Newburgh,

Aberdeenshire AB41 6AA 3Instituto Español de Oceanografía, Centro Oceanográfico de Vigo, Apdo. 1552, 36208 Vigo, Spain

4Loughs Agency, Dundalk Street, Carlingford, Co. Louth, Rep. Of Ireland

Executive Summary

1. The abundance of harbour seals in Carlingford Lough in the summer was reported previously

from a series of counts in 2008–11, to be 178–187 seals during the July pupping season and

350–375 seals during the moulting season from mid-August to early September. About 70% of

these seals haul out on Green Island and Mill Bay, both sites being close to the Black Hole

salmon rod and line fishery. Concern has therefore been expressed by the fishery that the seals

may be threatening the salmon stocks.

2. In order to investigate the diet of these seals during their period of peak numbers, a total of 59

scats collected between mid-August and early September 2009 & 2010 was analysed.

3. The diet was found to consist principally of small gadoid fish, such as cod, haddock and whiting,

and also flatfish such as flounder and plaice, and dragonet. All these types of fish have relatively

low energy density. The remains of relatively high energy fish, such as herring, sandeel,

mackerel and garfish, were occasionally found.

4. No salmonid remains were detected in any of the samples, although it should be borne in mind

that salmon otoliths and bones are relatively fragile and consumption of a small number of

salmon could thus go undetected. Nevertheless, from the completely negative results obtained

during August–September, it seems likely that the seals at Green Island and Mill Bay are not

targeting salmon to any significant extent during this season. The primary reason that harbour

seals assemble on Green Island in late summer is most probably due to its suitability, due to

long exposure time in the tidal cycle, for resting during their annual moult.

5. Recommendations for a future study include behavioural observations of seals in the water in

the vicinity of the Black Hole fishing area and the Whitewater river (since predation on large fish

such as adult salmon can be quantified by visual observation), a larger sample size of scats (to

increase the likelihood of detecting minor components of the diet) at critical salmon seasons,

and DNA analysis of scat sub-samples – which can increase the detection of salmonid-positive

scats.

6. Introduction

Carlingford Lough lies on the east coast of Ireland, at the southern extremity of the Mourne

Mountain range and straddling the border between Northern Ireland and the Republic of Ireland

(Fig. 1a). Two species of seal, the harbour (common) seal Phoca vitulina and the grey seal

Halichoerus grypus inhabit the coastline between Strangford Lough and the Ards Peninsula to the

north and Dublin Bay to the south. Carlingford Lough lies approximately in the middle of this area,

and in 2002–03 was thought to hold around 25% of about 1300 harbour seals and 10% of 350 grey

seals in the area (see Cronin et al., 2004; 2007). Seals of both species haul out on tidal rocks mainly

in the central and outer part of the Lough (Figure 1b).

Harbour seal abundance in Carlingford Lough has been estimated by the bounded count method to

be 178 individuals plus 54 newborn pups in July 2009 and 187 seals plus 43 newborns in July 2011.

Abundance was higher in August with estimated abundance of 350 and 375 harbour seals in 2008

and 2011 respectively. Grey seal abundance in Carlingford Lough was estimated at 39 and 55 seals

throughout the summers of 2009 and 2011 respectively (Wilson et al., 2012). Carlingford has not

been surveyed in the grey seal autumn breeding season, but it is believed that pup births

occasionally occur.

During seal surveys in 2008–11, attention was drawn to a perception among local salmon anglers of

seals having an adverse effect on the salmon and trout rod fishery in Carlingford Lough. The

Whitewater river contains a stock of salmon and sea trout. The Kilkeel Angling Club also release un-

fed fry derived from indigenous brood stock. Apart from rod angling in the river itself, anglers target

sea-trout and salmon in an area of Mill Bay near Greencastle known locally as the Black Hole (Figure

1c). Salmonids often gather in this area to await favourable water conditions for running the river.

Observations by local people of large numbers of seals, particularly at Green island in August–

September, have resulted in the perception that seals are increasing in number and are targeting

salmon, thus threatening the rod fishery and the salmon population in the Lough.

The purpose of this study was to obtain preliminary information on the diet of seals in the Green

Island and Mill Bay area during the period of maximum visible seal presence (the August moulting

period for harbour seals) with the aim of obtaining evidence of the extent of seal predation on

salmon and sea trout.

Animals and methods

Background: seal surveys of Carlingford Lough

Boat surveys of Carlingford seals during the summers 2008−11 have been previously reported

(Wilson et al., 2012). The present reviews a summary of these surveys to highlight the numbers of

seals at the sites of particular interest close to the Black Hole salmon rod fishery. Figure 2 shows a

typical boat survey route followed (excluding one site further north (Ballyedmond) surveyed from

the shore). Table 1 shows the distribution of harbour and grey seals at different haul-out sites

between mid-August and early September, which is the moulting season for harbour seals.

(a)

(b)

(c)

Figure 1. (a) the location of Carlingford Lough, wih the Ramsar Wetland site marked (total site area 8.270 km2, http://www.mpaglobal.org/index.php?action=showMain&s ite_code=220048), (b) the area of the Lough where harbour and grey seals occur at low tide, and (c) the diet study area, showing the location of Green Island and

Mill Bay haul-out site and scat sampling locations , the ‘Black Hole’ angling area and the Whi tewater river.

Figure 2. Seal survey area at low tide, 2008−11

Table 1. Median seal counts at low tide in Carlingford Lough mid-August to early September

2008−11 (n= 11 surveys; range of counts given in brackets)

Black Rock Carriganeen Mill Bay Green Isl Blockhouse Isl Greenore

Harbour seals 14 (3–23) 18 (3–32) 19 (0–85) 155 (69–204) 42 (1–75) 4 (0–19)

Grey seals 1 (0–7) 0 (0) 0 (0–1) 16 (8–22) 9 (0–18) 0 (0–2)

Greenore rocks

Green Island

Carriganeen

Black Rock

Mill Bay

grey seals

harbour seals

harbour seals

Blockhouse

Island

GREENCASTLE

CARLINGFORD

Green Island was the most ‘popular’ seal haul-out site in August and late September over the study

period (Table 1). Seals were found on all parts of the island, but most harbour seals (median 153

individuals) usually hauled out at the Mill Bay (north) end on the Greencastle aspect, while most

grey seals (median 12 individuals) hauled out in a tight group at the south end of the island on the

Carlingford aspect (Figure 2). Diet investigation in this study therefore focused on seals hauling out

at the north end of Green Island (where individuals were mainly – 97% on average – harbour seals).

Diet investigation methods

Background to methodology

The traditional method for investigating seal diet is the analysis of faecal samples collected at the

haul-out site (Pierce et al., 1991a). Harbour seal diet has been analysed from scats in many recent

studies, e.g. Pierce at al. (1991b); Olesiuk (1993); Brown & Pierce (1998); Tollit et al. (1997a); Hall et

al. (1998) and Wilson et al. (2002). The species of prey eaten by seals can be identified and

quantified using their hard remains (fish otoliths (earstones) and bones, cephalopod beaks

(mandibles) and crustacean exoskeletons extracted from each scat sample.

This method of diet analysis has the advantage of being non-invasive and inexpensive. There are also

some disadvantages in that the hard parts of different types of fish, particularly the otoliths, are not

equally robust in their survival of passage down the seal digestive tract, and therefore the remains of

some types of fish actually eaten by seals may be selectively eroded or totally digested and

therefore under-represented in faecal samples (e.g. Cottrell et al., 1996; Tollit et al., 1997b).

Bowen (2000) carried out a re-analysis of published data on feeding experiments to derive ‘number

correction factors’ (NCFs) for different species. NCFs were calculated as the inverse of the proportion

of the number of otoliths/beaks fed that were recovered in faecal samples, e.g. average NCFs for

haddock and whiting were approximately 1.1 and 1.3 respectively, whereas sandeels were 3.6,

herring−3, sprat−4.8, plaice−1.6, chinook salmon−1.6, and octopus−1.2. Bowen (op. cit.)

recommends that these correction factors be applied when carrying out diet analysis using otoliths

and beaks from faecal samples. However, applying NCFs can only work when there is a large enough

sample size to ensure the detection of at least one otolith, and this can be problematic for rare

species in the diet such as salmonids (Middlemass & Mackay, 2004). These authors recommend that

key skeletal structures (such vertebrae and pre-maxillae) also be used to detect salmonids, although

these may only give a non-quantifiable indication of presence or absence in a faecal sample, and

salmon bones are relatively friable also, and recognisable remains may not always be recovered.

Boyle et al (1990) considered that salmonid bone identification did not improve the incidence of

salmonid remains in seal faeces. A further problem exists if adult salmon are eaten, since if the head

is not consumed, the only potentially identifiable hard parts will be vertebrae, and these may be too

friable to survive passage through the digestive tract.

Using PCR techniques to detect species DNA in the soft part of scats may be a practical method to

supplement otolith and bone analysis from the hard parts in faecal samples: trials have indicated

that the incidence of detection increased by up to ~15% for salmonids, to ~10% for cephalopods and

to ~15% for flounder (Tollit et al., 2009). However, DNA analysis was not available for the present

study.

Another possible technique is the detection of salmonid proteins in faecal soft parts by serology

(Pierce at al., 1990a,b; Boyle et al., 1990). However, this was more successful using digestive tract

remains rather than faeces, and therefore involves catching animals and possibly using stomach

lavage. Another technique which involves catching animals is analysing fatty acid signatures in

blubber core samples (Iverson et al., 1997). These techniques could be considered in the context of

seals being caught for other purposes (eg for attaching telemetry tags), but were not practical for

the present study.

The present study has therefore been carried out using only traditional methods of scat

collection and analysis for otoliths and bones, in full recognition of the limitations of such hard-

part analysis.

Scat collections

Scats were collected mostly from Green Island (54° 2.109’N 06° 6.881’W; Fig 1c). 19 scats were

collected there on September 08 2009, a further 19 on August 16 2010 and 17 on August 31 2010.

An additional 4 scats were collected from the Mill Bay haul-out site (54° 2.642’N 06° 6.881’W; Figure

1c), also on August 16 2010.

Scat processing

Scats were softened in water with added laundry detergent, and then eased with tap water through

a 0.5mm sieve where the cleaned hard parts were collected. The hard parts were left to dry. Once

dried, recognisable hard parts were picked out with forceps under an illuminated lens and placed in

a numbered sample pot. For all samples all otoliths were retained. For the 2009 samples a

representative selection of vertebrae and other recognisable bones were retained, but for the 2010

samples all, or almost all vertebrae and other recognisable bones was retained. The samples were

kept dry in the sample pots until identification.

Prey identification and quantification

Each prey taxa possesses distinctive otoliths/bones/beaks and published guides are available to aid

identification (e.g. Härkönen, 1986, Watt et al., 1997). We also made use of the reference collections

of fish otoliths and bones and cephalopod beaks held at the University of Aberdeen. Otoliths

identifiable to family but not to species were grouped together as unidentified gadidae,

pluronectidae, clupeidae or cottidae. A small number of otoliths were too eroded to be identified to

even family level and were grouped as unidentified fish together with individuals quantified in

samples by the presence of remains that did not allow further identification such as fish eye lenses

and eroded bone fragments.

The number of fish was estimated from the number of otoliths or specific jaw bones (e.g. premaxilla,

preopercular, urohyal), whichever number was higher. Otolith number correction factors (NCFs,

Bowen, 2000) were not applied in this study, since not all otoliths were identifiable to species, NCFs

are not available for all species, e.g. dragonets, and it is not certain that such correction factors,

derived from captive studies on relatively inactive seals, should be applied in wild seals.

Fish sizes were estimated by measuring the otoliths, using a binocular microscope fitted with an

eyepiece graticule that was calibrated using a slide micrometer. For samples in which one fish

species was represented by >30 otoliths, a random sample of 30-60 otoliths was measured. Usually

otolith length was measured, except for the otoliths of herring , for which width is the standard

measurement (Härkönen, 1986), and any identifiable otolith that was broken lengthways. Fish length

and weight were calculated from standard regressions (e.g. Härkönen, 1986). For otoliths

identifiable to genus, family or other grouping of species, regressions based on combined data from

all the species in the group were used. To reconstruct total prey weight, each otolith was assumed to

represent 0.5 fishes. Thus, if both otoliths of an individual fish were present, the estimated weight of

this fish would be the average of the weights estimated separately from the two otoliths.

Correction factors to account for gastric erosion were not used since this approach (see Tollit et al.,

1997) involves correction based on visual grading of otoliths into digestion categories which has not

been found to produce always satisfactory results and therefore there are doubts concerning their

general applicability (see Tollit et al., 2010 for discussion) and there are no correction factors

available for all the species found in this study. However, it may be assumed that our results for fish

lengths and weights are slight underestimates.

Cephalopods beaks were also identified using reference material and guides (Clarke, 1986). Standard

measurements (hood length; Clarke, 1986) were taken on the lower beaks using a binocular

microscope fitted with an eyepiece graticule. Body weights of the animals were estimated using

standard regressions for lower beaks (Clarke, 1986).

Results

Prey species remains found in the faecal samples

The remains of 356 individual fish and 3 curled octopus (Eledone cirrhosa) were recovered from the

sieved scat samples in 2009 (Table 2) and 266 individual fish in 2010 (Table 3). Thirteen and fifteen

prey taxa were identified from these remains in 2009 and 2010, respectively.

Table 2. Overall importance of prey species identi fied from seal faeces in 2009 (N=19). The fi rs t es timate (%F) indicates the percentage

of faeces containing each prey category. The estimates for total number of individuals are based on (N 1) otoliths and beaks only and (N2) all prey remains . Measurements on otoli ths and beaks were used to derive the fi rst es timate of total prey weight (W1, g), while the

second estimate (W2, g) is adjusted to take account of fish identified from other remains. All four latter es timates are also expressed as percentages .

PREY SPECIES % F N1 N2 %N1 %N2 W1 W2 %W1 %W2

Fish 100 350 356 99.2 99.2 9410 9449 97.5 97.5 Herring (Clupea harengus) 21.1 4 4 1.0 1.1 347 347 3.6 3.6 Unidentified Clupeidae - - - - - - - - -

All Clupeoids 21.1 4 4 1.0 1.1 347 347 3.6 3.6 Cod (Gadus morhua) 21.1 49 49 13.3 13.6 2771 2771 28.7 28.6 Haddock (Melanogrammus aeglefinus) 10.5 1 1 0.3 0.3 132 132 1.4 1.4 Haddock/saithe/pollack 5.3 1 1 0.1 0.3 13 13 0.1 0.1 Whiting (Merlangius merlangus) 36.8 18 18 4.9 5.0 543 543 5.6 5.6

Trisopterus luscus/minutus 26.3 8 8 2.1 2.2 247 247 2.6 2.5 Trisopterus spp. (T. esmarkii, T. minutus, T. luscus) 31.6 5 5 1.4 1.4 65 65 0.7 0.7 Rocklings - - - - - - - - - Rockling/ling 10.5 4 4 1.0 1.1 70 70 0.7 0.7 Unidentified Gadidae 47.4 77 77 21.0 21.4 1023 1023 10.6 10.6

All Gadidae 68.4 163 163 46.1 45.3 4864 4864 50.4

50.2 Garfish (Belone belone) 5.3 2 2 0.6 0.6 729 729 7.6 7.5 Scorpaenidae - - - - - - - - - Bull-rout (Myoxocephalus scorpius) 5.3 1 1 0.3 0.3 73 73 0.8 0.8 Unidentified Cottidae - - - - - - - - -

All Cottidae 5.3 1 1 0.3 0.3 73 73 0.8 0.8 Labridae 5.3 2 2 0.6 0.6 35 35 0.4 0.4 Butterfish (Pholis gunnellus) - - - - - - - - - Sandeel (Ammodytidae) 15.8 43 43 12.2 12.0 462 462 4.8 4.8 Dragonet (Callyonymidae) 31.6 91 93 25.8 25.9 1770 1809 18.3 18.7

Mackerel (Scomber scombrus) 5.3 1 1 0.3 0.3 66 66 0.7 0.7 Plaice (Pleuronectes platessa) - - - - - - - - - Dab (Limanda limanda)? 5.3 1 1 0.3 0.3 159 159 1.6 1.6 Lemon sole (Microstomus kitt) - - - - - - - - - Witch (Glyptocephalus cynoglossus) - - - - - - - - -

Unidentified Pleuronectidae 47.4 31 31 8.8 8.6 906 906 9.4 9.3 All Pleuronectidae 47.4 32 32 9.1

8.9 1064 1064 11.0 11.0 Unidentified Fish 47.4 11 15 3.1 4.2 - - - -

CepCehalopodaC

Octopus (Eledone cirrhosa) 15.8 3 3 0.8 0.8 241 241 2.5 2.5

Table 3. Overall importance of prey species identi fied from seal faeces in 2010 (N=40). The fi rs t es timate (%F) indicates the

percentage of faeces containing each prey category. The estimates for total number of individuals are based on (N1)

otoliths and beaks only and (N2) all prey remains. Measurements on otoli ths and beaks were used to derive the fi rs t

es timate of total prey weight (W1, g), while the second estimate (W2, g) is adjusted to take account of fish identi fied from

other remains . All four latter es timates are also expressed as percentages .

PREY SPECIES % F N1 N2 %N1 %N2 W1 W2 %W1 %W2

Fish 100 242 266 100 100 12609 13227 100 100 Herring (Clupea harengus) 5.0

5 5 2.1 1.9 350 350 2.8 2.6 Unidentified Clupeidae - - - - - - - - -

All Clupeoids 5.0 5 5 2.1 1.9 350 350 2.8 2.6 Cod (Gadus morhua) 22.5 29 29 12.0 10.9 3852 3852 30.5 29.1 Haddock (Melanogrammus aeglefinus) 37.5 34 37 14.0 13.9 3542 3855 28.1 29.1 Haddock/saithe/pollack 10 9 9 3.7 3.4 725 725 5.7 5.5 Whiting (Merlangius merlangus) 12.5 9 9 3.7 3.4 361 361 2.9 2.7

Trisopterus luscus/minutus - - - - - - - - - Trisopterus spp. (T. esmarkii, T. minutus, T. luscus) 5.0 1 1 0.4 0.4 17 17 0.1 0.1 Rocklings - - - - - - - - - Rockling/ling 2.5 1 1 0.4 0.4 65 65 0.5 0.5 Unidentified Gadidae 22.5 22 25 9.1 9.4 626 711 5.0 5.4

All Gadidae 67.5 105

111 43.4 41.7

9189 9586 72.9 72.5 Garfish (Belone belone) - - - - - - - - - Scorpaenidae 2.5 1 1 0.4 0.4 17 17 0.1 0.1 Bull-rout (Myoxocephalus scorpius) - - - - - - - - - Unidentified Cottidae 2.5 3 3 1.2 1.1 18 18 0.1 0.1

All Cottidae 2.5 3 3 1.2 1.1 18 18 0.1 0.1 Labridae - - - - - - - - - Butterfish (Pholis gunnellus) 2.5 - 2 - 0.8 - - - - Sandeel (Ammodytidae) 7.5 13 13 5.4 4.9 192 192 1.5 1.5 Dragonet (Callyonymidae) 20 18 25 7.4 9.4 428 594 3.4 4.5

Mackerel (Scomber scombrus) - - - - - - - - - Plaice (Pleuronectes platessa) 2.5 1 1 0.4 0.4 71 71 0.6 0.5 Dab (Limanda limanda) 12.5 18 21 7.4

7.9 327 382 2.6 2.9 Lemon sole (Microstomus kitt) 2.5 - 1 - 0.4 - - - - Witch (Glyptocephalus cynoglossus) 5.0 2 2 0.8 0.8 171 171 0.1 1.3

Unidentified Pleuronectidae 27.5 49 49 20.2 18.4 1846 1846 14.6 14.0 All Pleuronectidae 40 70 74 28.8 27.8 2416 2470 19.2 18.7 Unidentified Fish 35 27 32 11.2 12.0 - - - -

Cephalopoda

Octopus (Eledone cirrhosa) - - - - - - - - -

Frequency of occurrence

Gadoids (cod, haddock and whiting among others) were the most frequent prey found in the

samples in both years, occurring in 68% of the samples. In 2009, the second most frequently found

prey (in 47% of the samples) were flatfish of the family Pleuronectidae although the poor stage of

the otoliths made it difficult to identify them to species level. Dragonets were also a common prey

found in 32% of the samples followed by herring (in 21% of the samples) and sandeels in 16%. The

curled octopus was found in 3 samples.

In 2010, again gadoids were the most frequent prey followed by flatfish (dab, plaice and lemon sole

that together appeared in 40% of the samples). Dragonets were found in 20% of the samples while

sandeel remains were recovered from 3 samples and herring from two. No cephalopod remains

were found in the 2010 samples.

Fig. 3. Percentage of samples containing remains of each prey category

Numbers of prey

Gadoids were the most common prey identified representing almost half of the total number of prey

recovered from the samples in both years. In 2009, dragonet was the second most numerous prey

group, comprising 26% of the total prey numbers followed by cod (13.6%) and sandeels (12%).

Cephalopods constituted less than 1% of the total number of prey (Table 2 and Figure 4).

In 2010 again Gadoids dominated the samples (42% of the total prey numbers) followed this time by

flatfish (28%), dragonets (9%) and sandeels (5%) (Table 3 and Figure 4).

Fig. 4. Overall percentage of each prey category occurring

Size of fish prey identified

Fish length

The median size of most fish was less than 200mm, with greater median lengths on for clupeids

(herring) in 2009 and cod in 2010 (Table 4). Maximum fish size exceeded 300mm only for cod

(maximum 339mm) and pleuronectids (maximum 310mm) (Table 4).

Table 4. Estimated l engths of main fish prey

2009 2010

species Median (mm) Range (mm)

Median (mm)

Range (mm)

Cod 145 106–339 211 134–326

Whiting 133 91–249 161 70–217

Trisopterus 141 30–208 137 132–142

Gadid (Other and Unid) 101 41–243 192 116–291

Dragonet 136 73–187 136 63–219

Pleuronectid 141 75–310 137 79–305 Clupeid 231 186–277 186 163–254

Sandeel 143 131–233 163 105–233

Prey weights

Gadoids (mainly cod and haddock) represented half of the reconstructed prey weight in 2009 (Table

2, Figure 5). In 2010 this percentage increased to almost ¾ of the total weight (Table 3, Figure 5).

Dragonet were more important by weight in the diet in 2009 (ca. 19% of the total weight) than in

2010 (less than 5% total weight). The contribution by weight to the diet of flatfish remained

relatively stable during both years with 11% of the total weight in 2009 and 14% in 2010. Clupeids

and sandeels were a relatively small component of the diet in both years (less than 5%) (Figure 5).

Fig. 5. The percentage of dietary components by weight

Discussion and recommendations

Discussion

The behaviour and diet of harbour seals during the annual moult

The results of this preliminary diet study indicate that seals – mainly harbour seals – hauling out on

Green island during the moulting season from mid-August to early September 2008 and 2011 were

feeding principally on small groundfish, mainly gadids (including cod, whiting, haddock, poorcod and

Norway pout), pleuronectids (mainly dab and others unidentified to species, probably flounder

and/or plaice) and also dragonets. The presence of curled octopus in the samples is also indicative of

benthic feeding. No salmonid remains were detected in these samples.

Most of the prey were small (less than 300mm) and were probably consumed whole underwater.

Hall et al. (1998) refer to an upper size limit of prey for harbour seals of ~300mm. The Green Island

seals’ diet at this season seems to have been deficient in oily fish, with herring accounting for less

than 4% by weight in both years.

Following the breeding season, European harbour (common) seals assemble on shore in relatively

dense concentrations in August and September while they undergo their annual moult over a 4–6

week period. During the moult the seals’ resting metabolic rate doubles due to increased blood flow

to the skin surface. The seals minimise the energetic cost of this by hauling out so as to maintain

optimal high skin surface temperature for hair growth (Paterson et al., 2012; Figure 6). Haul-out sites

favoured during the moult should be exposed during all or much of the tidal cycle, but not cut off

from water access. The north end of Green Island is therefore ideal for harbour seal moulting, and is

probably why seals concentrate here at this period.

Fig. 6. Surface temperature of a female harbour seal in captivity at 6 days post -partum (top) and during

the moult at 63 days post-partum (bottom) (From Paterson et al., 2012)

During much of the year, seals make foraging trips offshore often lasting several days. However,

during their moult the seals probably feed mainly on locally available prey in order to minimise long

sea trips and time spent underwater. The dietary components found in this study therefore probably

reflect prey found by seals either within or not far from Carlingford Lough.

The energetic content of cod, haddock and whiting is about 728–772 kcal kg-1, that of pleuronectids

dab, lemon sole and witch are 772–882 kcal kg-1, plaice 937 kcal kg-1 and octopus 688 kcal kg-1

(energy density values from various studies summarised and cited by Brown et al., 2001). By

contrast, the various energy-rich prey types were taken less often by these Green Island seals,

probably because of their local unavailability, namely herring (1850 kcal kg-1), sandeel (1173 kcal kg-

1), poorcod/pout (1102 kcal kg-1), mackerel (1839 kcal kg-1) and garfish (1575 kcal kg-1) (calorific

densities cited by Brown et al., 2001). The energetic content of dragonets has been quoted by Spitz

et al. (2010) for samples taken in the Bay of Biscay as 5.2 kJ g-1, which was half that of herring

measured in the same study. The authors also provided values for the curled octopus of 4.7 kJ g -1 ,

i.e. slightly lower than that of dragonets.

These figures for energy content suggest that the seals at Green Island in August-September are

subsisting on small, relatively low-energy marine groundfish, occasionally supplemented by higher

energy prey. This may be an energetically economic strategy for the moulting period compared with

making long foraging trips necessary to obtain higher quality prey. Salmon is also relatively high

energy fish, approximately double the energy density of cod (Olesiuk, 1993), and would probably

also be taken occasionally when available to the seals. However, it is not known if harbour seals

actively select high energy prey – they may just take prey in proportion to its relative abundance and

availability (Tollit et al., 1997a). This has given rise to the ‘junk food’ hypothesis, i.e. that seals will

eat too much low-quality fish when that is most abundant (Alverson, 1992; Rosen and Trites, 2000).

Evidence of macrocytic anaemia in harbour seals was recorded in the Moray Firth in ‘bad’ clupeid

years (clupeids <20% by weight in the diet), when gadoids predominated in the diet. This was

thought to be due to a heat-labile anti-metabolite in gadoids which reduces iron absorption and

decreased growth rates (Thompson et al., 1997). A low percentage of clupeids by weight also

occurred in the summer diet of seals in Dundrum Bay, just north of Carlingford Lough (Wilson et al.,

2002). However, it is not known whether the diet of the Green Island seals, found to be energy-

poor, clupeid-poor and gadoid-heavy, is simply a transient diet for the moulting period or reflects

year-round feeding. Studies at other times of the year would provide more data to address this

question.

Can salmon consumption by seals can be detected by traditional scat analysis?

This study of prey remains in seal faeces provided no support for the suggestion, by local anglers,

that seals assembling on Green Island during August-September are targeting salmon in Mill Bay.

However, the limitations of scat analysis for salmon detection are well known: seals may sometimes

discard the head of large salmon, and salmon bones are very friable and may not survive passage

through the digestive tract (Pierce et al., 1991b).

In several areas where harbour seals are known to prey on salmonids, evidence has indeed been

obtained from traditional scat sample analysis, with examples from British Columbia (Olesiuk, 1993),

the Moray Firth in Scotland (Pierce et al., 1991b Matejusova, 2008) and in Alaska (Geiger, 2011).

However, in these studies large numbers of scats were collected (in BC 2841 scats and in the Moray

Firth the two studies were based on 407 and 213 scats respectively), which would have increased

the chances of salmon detection. All these studies used other hard parts remains in addition to

otoliths to detect salmon presence in seal faeces. In BC, salmonid remains were found in ~15%

samples and were estimated to comprise an average of 3.1% of the diet and to be eaten in all

months of the year. In Alaska salmonids were estimated to contribute up to 0.6% of the diet

biomass, according to year. In the second Moray Firth study, up to 15% of scats collected in May and

July contained salmonid bones, although otoliths were only sometimes found.

Harbour seals feeding on salmon in captivity may sometimes discard the head (and therefore the

otoliths) of adult salmon, but at least sometimes eat the entire fish, including the head (Boyle et al.,

1990; Treacy, 1985). Two adult harbour seals were observed eating ‘black’ salmon at the mouth of

the River Thurso in April 2002. Both seals ate the entire fish, including the head. One seal took ~20

minutes to eat a fish that was ~60cm long, and the other took ~7 min to consume a smaller fish

(Wilson & Knight, 2002).

It is therefore clear that the presence of salmonids in the diet can be detected by traditional scat

analysis of otoliths and bones combined (an approach that we have followed), but probably a higher

number of scat samples would be needed to ascertain if salmon is taken by harbour seals in our

study area.

Potential alternative methods for detecting seal predation on salmon

There are two practical and non-invasive methods recently used to detect levels of seal predation on

salmonids. One method involves analysing scats at the molecular level, for salmonid DNA, using PCR

(polymerase chain reaction); the other involves monitoring the behaviour of seals in the vicinity of

salmon rivers by direct observation.

DNA analysis

PCR analysis of seal or sea lion scats for salmonid DNA may be done by analysing faecal remains

(Matejusova et al., 2008; Tollit et al., 2009). These studies took a subsample of each scat collected

for DNA analysis, while the remainder of the scat was processed for analysis of hard parts by the

conventional method. This DNA technique consistently resulted in ~10% of seal scats sampled in the

Moray Firth testing positive for salmonid DNA, which was higher than scats testing positive using

conventional hard-part analysis alone, although quantification of the amount of salmon residue in

DNA-positive scats resulted in very low concentrations being determined (Matejusova et al., op. cit.).

The DNA technique resulted in a 15% increase in salmonid detection in scats of Steller sea lions

(Eumetopias jubatus) in Alaska (Tollit et al., op. cit.).

Direct observation

Many rivers and estuaries are frequented by seals, particularly harbour seals, and this has given rise

to the general assumption by salmon fisheries and angling communities that seals are having a major

impact on local salmon stocks and local angling. On the basis that more objective assessment was

needed, a study of the behaviour of seals in the estuaries of two salmon rivers (the Dee and Don) in

north-east Scotland was carried during two separate 12-month periods and quantified in 1-hr

observation blocks (Carter et al., 2001). Seals were recorded as ‘hauled-out’, ‘in the water’, or

‘feeding’ (at the surface). In the last case, the prey was classed as salmonid, roundfish, flatfish or

‘other’. Most observed feeding events were of salmonids, with the highest number between

September–January and in May. Bootstrap simulations were carried out from the results to estimate

the number of salmonids taken over the year. This resulted in annual estimates of predation on large

salmonids by seals of ~8–10% of the number of salmonids caught by rod and line in the same areas

(Carter et al., 2001).

Conclusions

The main conclusion from this preliminary study of seal diet in Carlingford Lough is that there is no

evidence so far to support the assertion that the seals on Green Island and Mill bay in late summer

are targeting salmonids to any significant extent. These seals are, in fact, undergoing their annual

moult, which takes place from mid-August to early September. They have undoubtedly selected

Green Island as a suitable moulting site because of its availability at all states of the tide rather than

because of its proximity to the Black Hole salmon angling area. The moulting season is in any case

probably too early to coincide with the start of the adult salmon inward migration run, which is from

mid-September to December.

However, the results did suggest that these seals in the later summer moulting season were

subsisting on a relatively energy-poor diet of gadoids, flatfish and dragonets, supplemented by only

occasional oil-rich pelagic fish. If this poor diet should also be typical of seals hauling out in

Carlingford in the autumn and winter, seals might be tempted by salmonids when these are

relatively abundant – and possibly more vulnerable to seal predation when they are slower-moving

in winter. The Moray Firth study has suggested that harbour seal predation on sea trout is linked to

predation on flatfish, which was found to be relatively high in this study (Matejusova et al., 2008).

The occurrence of this prey grouping is linked to the seals’ use of different water depths and seabed

sediments. However, at present we have no information on seal haul-out numbers and diet in

Carlingford during autumn, winter and spring.

Recommendations This study was initiated because of the feeling amongst local anglers that the seals hauling out on

Green Island and Mill Bay in late summer were a threat to salmon moving between the Whitewater

river and the sea. The results to date show no predation on salmon. However, given the relatively

small sample size available for scats, and in the l ight of experience of other studies quoted above,

we suggest that further study would be useful, and might include the following components:

Seal counts at Green Island and Mill Bay throughout the year Behavioural observations, using digital camera, at the Black Hole and at the mouth of the

Whitewater river, particularly during the adult salmon and smolt runs Further diet studies by conventional scat analysis at Green Island and Mill Bay at different

seasons, specifically to include the period of the adult salmon run from late September to December and during the smolt run in April. Scat samples should be collected from both harbour and grey seal haul-out groups.

If possible, carry out DNA analysis for salmonids on a sub-sample of each scat

Acknowledgements This study has been the result of teamwork. We are indebted to all our colleagues at the Loughs

Agency, including Damien O’Malley, Donal Cassidy, Steven Moates, Dawn Hynes and Hannah

Cromie, who have willingly done the job of collecting the scat samples; in addition Sarah McLean and

Matt Kenrick have participated in seal counts. We would also like to thank Ashleen Higgins and Chris

McKnight for their assistance with preliminary scat analysis. We are grateful to John McCartney for

supporting this project intiative. Funding for this project was provided by the Loughs Agency.

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