Phocanema decipiens: Growth,reproduction, and survival in seals

Captive harbor seals (Phoca vitulina) and gray seals (Halichoerus grypus) were fed infective larvae of Phocanema decipiens, an anisakine nematode from the flesh of Atlantic cod (Gadus morhus). Ova of P. decipiens were first detected in the feces of harbor seals 21(17–30) days after exposure; the patency period was 15 to 45 days. In gray seals, the prepatent period was 19(16–23) days; patency 20–60 days. By the sixth week of infection in harbor seals, mean body lengths of adult females and males of P. decipiens were 60.8(40.8–76.2) and 54.3(45.5–60.8) mm, respectively; mean fecundity of female nematodes was 156,000 ova. In infections of similar duration in gray seals, females and males of P. decipiens were 82.1(69.7–104.3) and 64.4(53.8–72.7) mm in length, respectively; mean fecundity of females was 366,000 ova. In sensitizing infections in harbor seals, 28% of P. decipiens survived to early patency (Days 25–30) while only 9% of the nematodes survived to midpatency (Days 35–45). In sensitizing infections in gray seals, 56% of P. decipiens survived to early patency (Days 20–30) and 48% survived to midpatency (Days 35–50). Seals with existing or recent P. decipiens infections resisted reinfection; <50% of the nematodes in challenge infections in gray seals survived to Day 3 and <10% survived to patency. Growth of the nematodes, however, was not retarded in the challenge infections and resistence to reinfection subsided when seals were maintained anisakinefree for 2–6 months after loss of prior natural or experimental infections. Natural anisakine infections were surveyed in 16 harbor and 53 gray seals from the Nova Scotia mainland. The mean incidence of P. decipiens was 62(5–177) in harbor seals and 577(11–1694) in gray seals; incidence varied seasonally and with age of host. Adult females of P. decipiens from harbor seals were 64.0(49.2–79.8) mm in length and contained 1.68(0.87–2.73) × 10⁵ ova; females from gray seals were 78.3(62.3–92.1) mm in length and contained 2.39(0.69–4.39) × 10⁵ ova.     

Phocanema decipiens: Molting in seals

Infective larvae of the anisakine nematode Phocanema decipiens from the muscle of Atlantic cod (Gadus morhua) were fed to harbor seals (Phoca vitulina) and gray seals (Halichoerus grypus). During maturation in the stomach of seal hosts, P. decipiens molted twice; these molts are the third and fourth of its life cycle. The third molt occurred between the second and fifth days of infection. The third stage, i.e., infective larva entering the third molt, had a cuticular tooth ventral to the mouth; the fourth stage larva emerging from the third molt had three bilobed lips with dentigerous ridges. The fourth molt occurred between the 5th and 15th days in seals. A female nematode emerging from the fourth molt possesses a vulva and a vagina; a male possesses caudal alae, pre- and postanal papillae. Significant morphometric changes in nematodes were associated with both molts. Females and males of P. decipiens reached maturity after 15 to 25 days in seals. Ova were detected in the feces of the seal hosts as early as the 16th day.     

Helminth parasites in ringed seals (Pusa hispida) from Svalbard, Norway with special emphasis on nematodes: variation with age, sex, diet, and location of host

Complete gastrointestinal tracts from 257 ringed seals (Pusa hispida) from Svalbard, Norway, were examined for helminth parasites. Three different helminth groups were recorded (acanthocephalans 61.1%; nematodes 38%; cestodes 0.9%). Acanthocephalans (Polymorphidae) and cestodes (Anophryocephalus and Diphyllobothrium sp(p)., as well as unidentified species, were confined to the intestines. The anisakid nematodes Phocascaris phocae, Pseudoterranova sp(p)., Anisakis sp(p)., and Phocascaris/Contracaecum sp(p). were recorded in both stomachs and the anterior part of the small intestines. The abundance of nematodes and acanthocephalans varied significantly with sampling location of the seal hosts. This is likely due to the relative prevalence of Arctic versus Atlantic water in the different fjord systems, which strongly influences the age class and species of fish available as prey for the seals. Adult male ringed seals had significantly higher abundances of nematodes than did adult females or juveniles. Adult males also had significantly higher abundances of acanthocephalans than did adult females, but were not significantly different from juveniles in this regard. Nematode abundance increased significantly with age of male hosts, but this trend was lacking in female seals. Infection parameters appeared to be related to differences in the age of polar cod (Boreogadus saida) exploited by male, female, and juvenile seals.     

Phocanema decipiens: Intestinal penetration in the laboratory rat

Larval Phocanema decipiens from cod muscles were fed singly or repeatedly to Sprague-Dawley rats and the condition of the rats was monitored. Rats given two larvae each week for 10 weeks were considered to be sensitized. “Sensitized” and “naive” rats were each exposed to larvae during laparotomy and the tissue pathology was examined from 5 hr to 14 days after penetration—no differences were found. This is evidence against the “two-hit hypothesis” of pathology resulting largely from prior sensitization of the host. The lesion was also comparable to that made in the intestine with a sterile pin. Tissue changes showed acute inflammation followed by monocyte infiltration and then fibrocytes and granulation tissue finally left a fibrotic scar. Penetration of larvae through exposed intestinal loops of anesthetized and laparotomized rats was described using a closed-circuit TV system. These records are related to potentiometric recordings of isolated larvae in which a mean mechanical force of 1 g was typical with a maximum of 3.5 g generated by the integrated movement of the whole body. A force of 10 to 12 g/mm² was needed to penetrate the mucosa, muscularis, and serosa. The movements of the host's intestine and the rhythmic probing of the larvae clearly facilitate penetration. The data are used to support the hypothesis that mechanical factors of the larval force and the strength of the intestine dominate the pathological potential of P. decipiens. It is concluded that P. decipiens presents a very small pathological potential to the intact human gastrointestinal tract.     

THE TROPHIC ROLE OF MARINE MAMMALS IN THE NORTHERN GULF OF ST. LAWRENCE

The trophic role of apex predators was evaluated in the northern Gulf of St. Lawrence ecosystem. An Ecopath model was developed for the period 1985–1987 prior to the collapse of commercially exploited demersal fish stocks in this area. Marine mammal trophic levels were estimated by the model at 4.1 for cetaceans, 4.4 for harp seals (Pagophilus groenlandicus), 4.7 for hooded seals (Cystophora cristata), 4.5 for gray seals (Halichoerus grypus), and 4.3 for harbor seals (Phoca vitulina). Harp seals were the third most important predator on vertebrate prey following large Atlantic cod (Gadus morhua) and redfish (Sebastes spp.). Different seal species preyed on different levels of the food chain. Harp seals preyed on most trophic groups, whereas larger seals, such as gray seals and hooded seals, mainly consumed higher trophic levels. The model suggested that apex predators had a negative effect on their dominant prey, the higher trophic level fish, but an indirect positive feedback on the prey of their preferred prey, mainly American plaice (Hippoglossoides platessoides), flounders, skates, and benthic invertebrates. Our results suggest that both marine mammals and fisheries had an impact on the trophic structure.     

Domoic acid exposure and associated clinical signs and histopathology in Pacific harbor seals (Phoca vitulina richardii)

Domoic acid (DA) is a potent neurotoxin that has caused strandings and mortality of seabirds and marine mammals off the California coast. Pacific harbor seals (Phoca vitulina richardii) are an abundant, nearshore species in California; however, DA exposure and toxicosis have not been documented for harbor seals in this region. To investigate DA exposure in harbor seals, samples were collected from free-ranging and stranded seals off California to assess exposure, clinical signs of toxicosis, and brain lesions in harbor seals exposed to DA. Domoic acid was detected in 65% (17/26) of urine samples collected from apparently healthy free-ranging seals, with concentrations of 0.4–11.7 ng/ml. Domoic acid also was detected in feces (2.4–2887 ng/g), stomach contents (1.4 ng/g; stranded only), milk (2.2 ng/ml; stranded only), amniotic fluid (9.7 ng/ml; free-ranging only), fetal meconium (14.6–39.8 ng/g), and fetal urine (2.0–10.2 ng/ml). Clinical signs indicative of DA toxicosis were observed in two live-stranded seals, and included disorientation, seizures, and uncoordinated movements. Histopathology revealed the presence of brain lesions consistent with DA toxicosis in two live-stranded seals, and one free-ranging seal that died during capture. Results indicated that harbor seals were exposed to DA, exhibited clinical signs and histological lesions associated with DA exposure, and that pups were exposed to DA in utero and during lactation via milk. Future investigation is required to determine the magnitude of impact that DA has on the health and mortality of harbor seals.     

EFFECTS OF MEAL SIZE ON OTOLITH RECOVERY FROM FECAL SAMPLES OF GRAY AND HARBOR SEAL PUPS

Recovered otoliths from pinniped feces provide valuable information on diet composition and prey size. We studied the effect of meal size on otolith recovery from the feces of one harbor and eight gray seal pups. Each of 11 experiments comprised a half-ration meal, a period of fecal collection, a 1.5-or double-ration meal again followed by a period of fecal collection. A significantly lower percentage of Atlantic herring otoliths were recovered from half-ration meals (25%± 12.5% in the harbor seal, 8.6%± 6.9% in eight gray seals) than from 1.5- or double-ration meals (62.5%± 3.1 % in the harbor seal, 32.8%± 23.5% in gray seals). Meal size also significantly affected the percentage of Atlantic cod otoliths recovered from gray seal feces (65.0%± 26.3% from half ration, 98.3%± 2.9% from 1.5 ration). For both size meals, recovered cod otoliths were more significantly eroded than herring otoliths. The development of correction factors to account for the effects of digestion will need to consider the distribution of meal sizes of free-ranging pinnipeds.     

Natural selection on marine carnivores elaborated a diverse family of classical MHC class I genes exhibiting haplotypic gene content variation and allelic polymorphism

Pinnipeds, marine carnivores, diverged from terrestrial carnivores ~45 million years ago, before their adaptation to marine environments. This lifestyle change exposed pinnipeds to different microbiota and pathogens, with probable impact on their MHC class I genes. Investigating this question, genomic sequences were determined for 71 MHC class I variants: 27 from harbor seal and 44 from gray seal. These variants form three MHC class I gene lineages, one comprising a pseudogene. The second, a candidate nonclassical MHC class I gene, comprises a nonpolymorphic transcribed gene related to dog DLA-79 and giant panda Aime-1906. The third is the diversity lineage, which includes 62 of the 71 seal MHC class I variants. All are transcribed, and they minimally represent six harbor and 12 gray seal MHC class I genes. Besides species-specific differences in gene number, seal MHC class I haplotypes exhibit gene content variation and allelic polymorphism. Patterns of sequence variation, and of positions for positively selected sites, indicate the diversity lineage genes are the seals’ classical MHC class I genes. Evidence that expansion of diversity lineage genes began before gray and harbor seals diverged is the presence in both species of two distinctive sublineages of diversity lineage genes. Pointing to further expansion following the divergence are the presence of species-specific genes and greater MHC class I diversity in gray seals than harbor seals. The elaboration of a complex variable family of classical MHC class I genes in pinnipeds contrasts with the single, highly polymorphic classical MHC class I gene of dog and giant panda, terrestrial carnivores.     

Postrelease movement of rehabilitated harbor seal (Phoca vitulina richardii) pups compared with cohort‐matched wild seal pups

Harbor seal (Phoca vitulina richardii) populations in the inland waters of Washington and British Columbia are at or near carrying capacity. Stranded pups often are collected and admitted to rehabilitation centers, and then released when they reach a weight of 22 kg and meet a variety of preestablished health and release conditions. While rehabilitation is common practice, it is unclear if rehabilitated seal pups behave like wild weaned pups. Using satellite transmitters, we compared movement patterns of 10 rehabilitated pups with 10 wild weaned pups. When released, rehabilitated seals were longer and heavier than wild pups, while wild pups had a larger mean axillary girth. No clinically different blood parameters were detected. On average, rehabilitated harbor seal pups traveled nearly twice as far cumulatively, almost three times as far daily, and dispersed over three times as far from the release site compared to wild weaned seals. Additionally, wild harbor seals transmitted nearly twice as long as did rehabilitated seals. These patterns suggest that learned behavior during the brief 3–4 wk nursing period likely enables wild harbor seal pups to move less daily and remain closer to their weaning site than rehabilitated pups.     

Occurrence of the tuna nematode <Emphasis Type="Italic">Hysterothylacium cornutum</Emphasis> (Stossich, 1904) in farmed Atlantic cod <Emphasis Type="Italic">Gadus morhua</Emphasis> L. in North Norway

Adult and fourth-stage larval nematodes found in the stomachs of farmed cod in North Norway in October 2006 were identified as Hysterothylacium cornutum (Stossich, 1904), a nematode considered to be specific at the adult stage to tunas of the genus Thunnus. As far as we are aware, this is the first report of an adult form of this nematode from any host other than members of the genus Thunnus, and is also the first report from a polar region. The two infected cod, one with one large adult worm and another with six larvae, were in a sample of 17 that had been captured from the wild about 1 year before sampling and held in floating sea cages in Øksfjord, Finnmark County, North Norway, where the examinations took place. No further infections were found in any other samples of wild and farmed cod, totalling 261 fish, examined by us during the period 2006 and 2007 from five locations distributed along the coast of Norway from Øksfjord in the north to Ålesund in the south. Possible sources of this unusual infection are discussed, but no firm conclusion could be reached.     

Transmission of lungworms of harbour porpoises and harbour seals: Molecular tools determine potential vertebrate intermediate hosts

Harbour porpoises (Phocoena phocoena) and harbour seals (Phoca vitulina) from German waters are infected by six species of lungworms (Metastrongyloidea). These nematodes parasitise the respiratory tract, are pathogenic and often cause secondary bacterial infections. In spite of their clinical and epidemiological significance, the life cycle and biology of lungworms in the marine environment is still largely unknown. Regions of ribosomal DNA (ITS-2) of all lungworms parasitising harbour porpoises and harbour seals in German waters were sequenced to characterise and compare the different species. The phylogenetic relationship among the lungworm species was analysed by means of their ITS-2 nucleotide sequences and the species-specific traits of the ITS-2 were used to screen wild fish as possible intermediate hosts for larval lungworms. Molecular markers were developed to identify larval nematodes via in-situ hybridisation of tissues of harbour porpoise and harbour seal prey fish. Potential wild intermediate fish hosts from the North Sea were dissected and found to harbour larval nematodes. Histological examination and in-situ hybridisation of tissue samples from these fish showed lungworm larvae within the intestinal wall. Based on larval ITS-2 nucleotide sequences, larval nematodes were identified as Pseudalius inflexus and Parafilaroides gymnurus. Turbot (Psetta maxima) bred and raised in captivity were experimentally infected with live L1s of Otostrongylus circumlitus and ensheathed larvae were recovered from the gastrointestinal tract of turbot and identified using molecular tools. Our results show that fish intermediate hosts play a role in the transmission of metastrongyloid nematodes of harbour porpoises and harbour seals.     

Seroprevalence of Antibodies against Seal Influenza A(H10N7) Virus in Harbor Seals and Gray Seals from the Netherlands

In the spring and summer 2014, an outbreak of seal influenza A(H10N7) virus infection occurred among harbor seals (Phoca vitulina) off the coasts of Sweden and Denmark. This virus subsequently spread to harbor seals off the coasts of Germany and the Netherlands. While thousands of seals were reported dead in Sweden, Denmark and Germany, only a limited number of seals were found dead in the Netherlands. To determine the extent of exposure of seals in the Netherlands to influenza A/H10N7 virus, we measured specific antibody titers in serum samples from live-captured seals and seals admitted for rehabilitation in the Netherlands by use of a hemagglutination inhibition assay and an ELISA. In harbor seals in 2015, antibodies against seal influenza A(H10N7) virus were detected in 41% (32 out of 78) pups, 10% (5 out of 52) weaners, and 58% (7 out of 12) subadults or adults. In gray seals (Halichoerus grypus) in 2015, specific antibodies were not found in the pups (n = 26), but in 26% (5 out of 19) of the older animals. These findings indicate that, despite apparent low mortality, infection with seal influenza A(H10N7) virus was geographically widespread and also occurred in grey seals.     

A missing piece in the Arctic food web puzzle? Stomach contents of Greenland sharks sampled in Svalbard, Norway

Harbour seals in Svalbard have short longevity, despite being protected from human hunting and having limited terrestrial predation at their haulout sites, low contaminant burdens and no fishery by-catch issues. This led us to explore the diet of Greenland sharks (Somniosus microcephalus) in this region as a potential seal predator. We examined gastrointestinal tracts (GITs) from 45 Greenland sharks in this study. These sharks ranged from 229 to 381?cm in fork length and 136–700?kg in body mass; all were sexually immature. Seal and whale tissues were found in 36.4 and 18.2%, respectively, of the GITs that had contents (n?=?33). Based on genetic analyses, the dominant seal prey species was the ringed seal (Pusa hispida); bearded seal (Erignathus barbatus) and hooded seal (Cystophora cristata) tissues were each found in a single shark. The sharks had eaten ringed seal pups and adults based on the presence of lanugo-covered prey (pups) and age determinations based on growth rings on claws (≤1?year and adults). All of the whale tissue was from minke whale (Balenoptera acutorostrata) offal, from animals that had been harvested in the whale fishery near Svalbard. Fish dominated the sharks’ diet, with Atlantic cod (Gadus morhua), Atlantic wolffish (Anarhichas lupus) and haddock (Melanogrammus aeglefinus) being the most important fish species. Circumstantial evidence suggests that these sharks actively prey on seals and fishes, in addition to eating carrion such as the whale tissue. Our study suggests that Greenland sharks may play a significant predatory role in Arctic food webs.     

Rugopharynx rosemariae new species (nematoda: Pharyngostrongylidae) from grey kangaroos (Macropus gig anteus and M. fuliginosus) with life cycle stages and associated pathology

Beverdge I. and Ppresidente P. J. A. 1978. Rugopharynx rosemarie sp. nov. (Nematoda: Pharyngostrongylidae) from grey kangaroos (Macropus giganteus and M. fuliginosus) with life cycle stages and associated pathology. International Journal for Parasitology8: 379–387. Rugopharynx rosemariae new species is described from the stomachs of grey kangaroos, Macropus giganteus Shaw, 1790 and Macropus fuliginosus (Desmarest, 1817) from south-eastern Australia. The new species differs from other species of the genus in spicule length and in the length and shape of the oesophagus. Parasitic life-cycle stages are described from natural infections; free-living life-cycle stages were obtained by culturing eggs from gravid females. Third stage larvae burrow into the gastric mucosa producing small elevated nodules resulting from a localized fibroplastic and inflammatory reaction in the lamina propria and submucosa. Fourth stage and adult worms occur in the stomach lumen. Rugopharynx brevis (Canavan, 1931) is made a synonym of Rugopharynx australis (Mönnig, 1926).     

DISPERSAL and BYCATCH MORTALITY IN GRAY, HALICHOERUS GRYPUS, AND HARBOR, PHOCA VITULINA, SEALS TAGGED AT THE NORWEGIAN COAST

Between 1975 and 1998, 3,571 gray and 630 harbor seal pups were tagged along the Norwegian coast, and 259 (7%) gray and 80 (13%) harbor seal tags were returned. Incidental mortality, mainly in bottom-set nets, accounted for the majority of deaths (79% in gray and 48% in harbor seals, respectively). Seals were most vulnerable to incidental mortality in fishing gear during the first three months after birth, but high incidental mortality prevailed during the first 8–10 mo. Gray seals dispersed more widely (mean distance: 120 km) than harbor seals (mean distance: 69 km). Both species dispersed most widely during the two first months after tagging. The maximum distance moved was 739 km for gray and 463 km for harbor seals. Strong fidelity for their place of birth was observed in adult gray seals during breeding season. No significant difference in incidental mortality was detected between the areas of tagging. However, for 37 harbor seals tagged in a 724 km nature reserve no returns were reported.     

The effect of prey quality and ice conditions on the nutritional status of Baltic gray seals of different age groups

We analyzed a long-term data set of the body condition of Baltic gray seals (Halichoerus grypus) over time and investigated how average subcutaneous blubber thickness of different age groups of seals corresponds to environmental factors. Blubber thickness of pups declined until 2010. The decreasing weight of 5–6-year-old herring (Clupea harengus), the main prey fish for Baltic gray seals, explained well the decline. In the Gulf of Finland, the blubber thickness of pups declined also in recent years (2011–2015) with declining number of days with permanent ice cover. In other regions, the blubber thickness of pups increased during recent years with increasing weight of herring. The blubber thickness of sub-adults in Baltic Proper and that of hunted adult females in the Bothnian Bay also increased during recent years, and the weight of age 6+ or 7-year-old herring best explained the increase. The blubber thickness of all age groups of seals was thinnest in the Bothnian Bay where also herring weight was lowest. There was a negative correlation between blubber thickness of seals and herring catch size (an index of herring abundance) suggesting that herring quality, not the quantity, is important for the nutritional status of Baltic gray seals. Nutritional status of gray seals may thus reveal changes in the marine food web which affect herring quality. Marine food web, in turn, may be affected, e.g., by climate change. The warming climate also has an impact on ice cover and thus body condition of seal pups.     

Molecular Epidemiology of Seal Parvovirus, 1988–2014

A novel parvovirus was discovered recently in the brain of a harbor seal (Phoca vitulina) with chronic meningo-encephalitis. Phylogenetic analysis of this virus indicated that it belongs to the genus Erythroparvovirus, to which also human parvovirus B19 belongs. In the present study, the prevalence, genetic diversity and clinical relevance of seal parvovirus (SePV) infections was evaluated in both harbor and grey seals (Halichoerus grypus) that lived in Northwestern European coastal waters from 1988 to 2014. To this end, serum and tissue samples collected from seals were tested for the presence of seal parvovirus DNA by real-time PCR and the sequences of the partial NS gene and the complete VP2 gene of positive samples were determined. Seal parvovirus DNA was detected in nine (8%) of the spleen tissues tested and in one (0.5%) of the serum samples tested, including samples collected from seals that died in 1988. Sequence analysis of the partial NS and complete VP2 genes of nine SePV revealed multiple sites with nucleotide substitutions but only one amino acid change in the VP2 gene. Estimated nucleotide substitution rates per year were 2.00×10⁻⁴ for the partial NS gene and 1.15×10⁻⁴ for the complete VP2 gene. Most samples containing SePV DNA were co-infected with phocine herpesvirus 1 or PDV, so no conclusions could be drawn about the clinical impact of SePV infection alone. The present study is one of the few in which the mutation rates of parvoviruses were evaluated over a period of more than 20 years, especially in a wildlife population, providing additional insights into the genetic diversity of parvoviruses.     

Novel B19-Like Parvovirus in the Brain of a Harbor Seal

Using random PCR in combination with next-generation sequencing, a novel parvovirus was detected in the brain of a young harbor seal (Phoca vitulina) with chronic non-suppurative meningo-encephalitis that was rehabilitated at the Seal Rehabilitation and Research Centre (SRRC) in the Netherlands. In addition, two novel viruses belonging to the family Anelloviridae were detected in the lungs of this animal. Phylogenetic analysis of the coding sequence of the novel parvovirus, tentatively called Seal parvovirus, indicated that this virus belonged to the genus Erythrovirus, to which human parvovirus B19 also belongs. Although no other seals with similar signs were rehabilitated in SRRC in recent years, a prevalence study of tissues of seals from the same area collected in the period 2008-2012 indicated that the Seal parvovirus has circulated in the harbor seal population at least since 2008. The presence of the Seal parvovirus in the brain was confirmed by real-time PCR and in vitro replication. Using in situ hybridization, we showed for the first time that a parvovirus of the genus Erythrovirus was present in the Virchow-Robin space and in cerebral parenchyma adjacent to the meninges. These findings showed that a parvovirus of the genus Erythrovirus can be involved in central nervous system infection and inflammation, as has also been suspected but not proven for human parvovirus B19 infection.     

Temperature,salinity tolerance,and buoyancy during early development and starvation of Clyde and North Sea herring,cod, and flounder larvae

Tolerance limits, at which 50% of larvae could survive high temperature and low salinity for 24 h, were determined for the yolk-sac larvae of Clyde and North Sea herring (Clupea harengus L.), cod (Gadus morhua L.) and flounder (Platichthys flesus L.) during early development and starvation. Clyde and North Sea herring, cod and flounder from hatching to the end of the yolk-sac stage, could withstand 21–23.5 °C, 20.5–23 °C, 15.5–18 °C and 21.5–24°C, respectively. The temperature tolerance was reduced by about 3.5–4 °C for Clyde herring and cod, 4–4.5 °C for North Sea herring and 8–8.5 °C for flounder when the larvae reached the point-of-no-return (PNR, when 50% of larvae, although still alive, are no longer strong enough to feed). The lowest salinity tolerance between hatching and the end of yolk-sac stage was 1–1.5‰ for Clyde and North Sea herring, 2–3‰ for cod and 0–1‰ for flounder. In no instance was there a loss of tolerance to low salinity during starvation. In fact, tolerance improved somewhat until the larvae became moribund. At hatching Clyde and North Sea herring larvae were negatively buoyant with a sinking rate of 0.35–0.4cm · s⁻¹ which steadily decreased until the larvae became moribund. Cod and flounder larvae, however, were positively buoyant at hatching but became progressively less buoyant and, by the end of the yolk-sac stage they were negatively buoyant with a sinking rate of 0.06–0.07 cm · s⁻¹. This sinking rate then decreased slightly until the PNR stage. The low salinity tolerance of all three species varied in a similar fashion to buoyancy.     

Predation by a carnivorous marine copepod,Euchaeta norvegica Boeck,on eggs and larvae of the North Atlantic cod Gadus morhua L.

The predatory behavior of a carnivorous marine copepod, Euchaeta norvegica Boeck, feeding on eggs and larvae of the North Atlantic cod Gadus morhua L. was examined. In the laboratory, adult females of Euchaeta norvegica did not feed on eggs. Predation rates on yolk-sac larvae and starved post-yolk-sac larvae did not vary significantly with age up to 14 days old because of little change in size or activity of the larvae. This differs from E. elongata Esterly, a temperate congener, which selectively feeds on middle yolk-sac-stage larvae of the Pacific hake Merluccius productus Ayres. The subarctic congener Euchaeta norvegica appeared to detect tailbeats of the cod larvae. The functional response was measured for E. norvegica feeding on 2–4-day-old yolk-sac larvae. Maximum ingestion was achieved at 5 larvae · 1⁻¹ with a rate of 6.3 ± 1.2 larvae·copepod⁻¹·day⁻¹ or 10.5% of its body weight. Estimates of short-term feeding rates, determined from gut-evacuation curves, indicate that E. norvegica, when preying on cod larvae only, must feed for at least 4 h to achieve this maximum ingestion rate. Presence of copepods as alternative prey for E. norvegica depresses its predation rate on cod, although the ingestion of cod greatly supplements the ration consumed. Copepods fed cod larvae form black melanin-pigmented fecal pellets in which larval cod otoliths have been found. Approximately 0.5 larva was required to form one fecal pellet. The last three developmental stages of the predatory copepod were able to ingest larvae and form dark-pigmented fecal pellets. The feeding of this carnivorous marine copepod may contribute to the mortality noted in the larval stages of cod because E. norvegica is numerous in the center of the cod-spawning area of Skrova in the Lofoten Islands, northern Norway.