The abundance and distribution of Lepeophtheirus salmonis (Copepoda: Caligidae) on pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon in coastal British Columbia

In total, 23,750 specimens of the salmon louse, Lepeophtheirus salmonis, were collected from 3,907 juvenile pink and 3,941 chum salmon caught within the Broughton Archipelago during a 2-yr survey. The prevalence on pink salmon was significantly higher than on chum salmon in 2004 (62.3% and 58.6%, respectively) and in 2005 (26.4% and 23.1%, respectively). The mean abundance on chum salmon was significantly higher than on pink salmon in 2004 (7.0 +/- 0.3 and 2.8 +/- 0.2, respectively), whereas in 2005 the mean abundance did not differ between species (0.6 +/- 0.1 and 0.5 +/- 0.0, respectively). The mean intensity on chum salmon was significantly higher than on pink salmon in 2004 (12.0 +/- 0.4 and 4.5 +/- 0.2, respectively) and in 2005 (2.5 +/- 0.2 and 1.7 +/- 0.1, respectively). The prevalence, intensity, and abundance of L. salmonis were significantly higher on salmon belonging to both host species in 2004 compared with 2005. In both years, a majority of pink and chum salmon had 2 or fewer lice. In general, a decline in abundance of L. salmonis over the 3 collection periods in each year coincided with an increased percentage of motile developmental stages. The abundance was lowest on fish collected from zones in which the seawater surface salinity was also lowest. Seawater surface temperature was higher and salinity was lower in 2004 compared with 2005. The spatial and temporal trends in the abundance of L. salmonis in relation to host size, infestation rates, and seawater salinity and temperature, evident in both years, must be considered in future studies assessing the role of farmed salmon in the epizootiology of this parasite on juvenile salmon in this area.     

Differential expression of the pro-inflammatory cytokines IL-1beta-1, TNFalpha-1 and IL-8 in vaccinated pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon juveniles

Laboratory-reared pink and chum salmon juveniles (approximately 2g) received an intraperitoneal injection with a commercial, unadjuvanted Aeromonas salmonicida bacterin or sterile saline. Relative to elongation factor-1A, expression levels of genes encoding the proinflammatory cytokines interleukin-1beta-1 (IL-1beta), tumour necrosis factor-alpha-1 (TNFalpha) and interleukin-8 (IL-8) in pools of kidney and liver were examined 6- and 24-h after injection. Expression of IL-1beta was significantly elevated in pink and chum salmon by 6-h, and declined in pink salmon but not in chum salmon by 24-h. Similarly, expression of TNFalpha was significantly elevated in both species at 6h and only in chum salmon after 24-h. Expression of IL-8 was significantly elevated in both species at 6- and 24-h after injection. Expression of the three proinflammatory cytokine genes differed between salmon species both in the timing and magnitude of their expression. The significance of these differences with respect to immune function in these fish requires further research.     

Effects of host migration, diversity and aquaculture on sea lice threats to Pacific salmon populations

Animal migrations can affect disease dynamics. One consequence of migration common to marine fish and invertebrates is migratory allopatry-a period of spatial separation between adult and juvenile hosts, which is caused by host migration and which prevents parasite transmission from adult to juvenile hosts. We studied this characteristic for sea lice (Lepeophtheirus salmonis and Caligus clemensi) and pink salmon (Oncorhynchus gorbuscha) from one of the Canada's largest salmon stocks. Migratory allopatry protects juvenile salmon from L. salmonis for two to three months of early marine life (2-3% prevalence). In contrast, host diversity facilitates access for C. clemensi to juvenile salmon (8-20% prevalence) but infections appear ephemeral. Aquaculture can augment host abundance and diversity and increase parasite exposure of wild juvenile fish. An empirically parametrized model shows high sensitivity of salmon populations to increased L. salmonis exposure, predicting population collapse at one to five motile L. salmonis per juvenile pink salmon. These results characterize parasite threats of salmon aquaculture to wild salmon populations and show how host migration and diversity are important factors affecting parasite transmission in the oceans.     

Functional response of juvenile pink and chum salmon: effects of consumer size and two types of zooplankton prey

Feeding rate experiments were conducted for pink salmon Oncorhynchus gorbuscha fry [mean fork length ( L _F) 39 mm], juveniles (103–104 mm L _F) and juvenile chum salmon Oncorhynchus keta (106–107 mm L _F). Fishes were presented with small copepod ( Tisbi sp.) or larger mysid shrimp ( Mysidopsis bahia ) prey at varying densities ranging from 1 to 235 prey l⁻¹ in feeding rate experiments conducted at water temperatures ranging from 10·5 to 12·0° C under high light levels and low turbidity conditions. Juvenile pink and chum salmon demonstrated a type II functional response to mysid and copepod prey. Mysid prey was readily selected by both species whereas the smaller bodied copepod prey was not. When offered copepods, pink salmon fry fed at a higher maximum consumption rate (2·5 copepods min⁻¹) than larger juvenile pink salmon (0·4 copepods min⁻¹), whereas larger juvenile chum salmon exhibited the highest feeding rate (3·8 copepods min⁻¹). When feeding on mysids, the maximum feeding rate for larger juvenile pink (12·3 mysids min⁻¹) and chum (11·5 mysids min⁻¹) salmon were similar in magnitude, and higher than feeding rates on copepods. Functional response models parameterized for specific sizes of juvenile salmon and zooplankton prey provide an important tool for linking feeding rates to ambient foraging conditions in marine environments, and can enable mechanistic predictions for how feeding and growth should respond to spatial-temporal variability in biological and physical conditions during early marine life stages.     

Physiological consequences of the salmon louse (Lepeophtheirus salmonis) on juvenile pink salmon (Oncorhynchus gorbuscha): implications for wild salmon ecology and management, and for salmon aquaculture

Pink salmon, Oncorhynchus gorbuscha, are the most abundant wild salmon species and are thought of as an indicator of ecosystem health. The salmon louse, Lepeophtheirus salmonis, is endemic to pink salmon habitat but these ectoparasites have been implicated in reducing local pink salmon populations in the Broughton Archipelago, British Columbia. This allegation arose largely because juvenile pink salmon migrate past commercial open net salmon farms, which are known to incubate the salmon louse. Juvenile pink salmon are thought to be especially sensitive to this ectoparasite because they enter the sea at such a small size (approx. 0.2 g). Here, we describe how 'no effect' thresholds for salmon louse sublethal impacts on juvenile pink salmon were determined using physiological principles. These data were accepted by environmental managers and are being used to minimize the impact of salmon aquaculture on wild pink salmon populations.     

Transmission dynamics of parasitic sea lice from farm to wild salmon

Marine salmon farming has been correlated with parasitic sea lice infestations and concurrent declines of wild salmonids. Here, we report a quantitative analysis of how a single salmon farm altered the natural transmission dynamics of sea lice to juvenile Pacific salmon. We studied infections of sea lice (Lepeophtheirus salmonis and Caligus clemensi) on juvenile pink salmon (Oncorhynchus gorbuscha) and chum salmon (Oncorhynchus keta) as they passed an isolated salmon farm during their seaward migration down two long and narrow corridors. Our calculations suggest the infection pressure imposed by the farm was four orders of magnitude greater than ambient levels, resulting in a maximum infection pressure near the farm that was 73 times greater than ambient levels and exceeded ambient levels for 30 km along the two wild salmon migration corridors. The farm-produced cohort of lice parasitizing the wild juvenile hosts reached reproductive maturity and produced a second generation of lice that re-infected the juvenile salmon. This raises the infection pressure from the farm by an additional order of magnitude, with a composite infection pressure that exceeds ambient levels for 75 km of the two migration routes. Amplified sea lice infestations due to salmon farms are a potential limiting factor to wild salmonid conservation.     

Effect of environmental salinity on sea lice Lepeophtheirus salmonis settlement success

The sea louse Lepeophtheirus salmonis (Kr?yer, 1837) (Copepoda: Caligidae) is an ectoparasite of salmonid fish. It has earlier been proposed that the free-swimming infectious copepodid stage of L. salmonis gather at river mouths to infect wild Atlantic salmon Salmo salar L. and sea trout S. trutta L. smolts during their seaward migration. This study used aquarium-based methods to investigate the survival, infective ability and behaviour of L. salmonis copepodids exposed to short periods of low salinity levels, such as those encountered at river mouths. Survival of free-swimming copepodids was found to be severely compromised at salinity levels below 29 parts per thousand (ppt). Attachment to an S. salar host did not aid copepodid survival during post-infection exposure to low salinity environment, and a reduction in salinity appears to reduce the ability of copepodids to remain attached to S. salar smolts. Pre-infection exposure of copepodids to reduced salinity levels reduced infection of S. salar. Infection levels at reduced salinity were lower than predicted from the free-swimming survival experiment, suggesting that low salinity compromises the copepodids' ability to sense or respond to the presence of a host. In salinity gradients, copepodids demonstrated avoidance of salinities below 27 ppt, by both altering their swimming behaviour and changing the orientation of passive sinking. Avoidance of low salinity levels may be due to their adverse effects on copepodid physiology, as suggested by the reduction in survival. Sinking rates were also faster in reduced salinity, suggesting that remaining in the water column would be more energetically demanding for the copepodids at reduced salinity. These results show that both survival and host infectivity of L. salmonis are severely compromised by short-term exposure to reduced salinity levels.     

Sea lice escape predation on their host

Parasites seldom have predators but often fall victim to those of their hosts. How parasites respond to host predation can have important consequences for both hosts and parasites, though empirical investigations are rare. The exposure of wild juvenile salmon to sea lice (Lepeophtheirus salmonis) from salmon farms allowed us to study a novel ecological interaction: the response of sea lice to predation on their juvenile pink and chum salmon hosts by two salmonid predators-coho smolts and cut-throat trout. In approximately 70% of trials in which a predator consumed a parasitized prey, lice escaped predation by swimming or moving directly onto the predator. This trophic transmission is strongly male biased, probably because behaviour and morphology constrain female movement and transmission. These findings highlight the potential for sea lice to be transmitted up marine food webs in areas of intensive salmon aquaculture, with implications for louse population dynamics and predatory salmonid health.     

Reduced Lepeophtheirus salmonis larval abundance in a sea loch on the west coast of Scotland between 2002 and 2006

A survey for planktonic sea louse larvae was carried out in Loch Shieldaig, Scotland, between 2002 and 2006, and spanned 2 successive production cycles (Cycles 1 and 2) at a local Atlantic salmon Salmo salar L. farm. The vast majority of the caligid copepodids recovered were Lepeophtheirus salmonis; however, the methodology was unable to determine the species of the caligid nauplii. Greatest densities of nauplii were found at the sampling station adjacent to the salmon farm, and larval densities were low during the fallow period of both cycles. Peaks in nauplius densities occurred around the same time in the 2 cycles, but the peaks were significantly lower during Cycle 2 than Cycle 1. Lepeophtheirus salmonis copepodid densities varied temporally, but not spatially. During most of Cycle 2, copepodid densities were significantly lower than those recovered during Cycle 1. Numbers of gravid L. salmonis at the local salmon farm correlated significantly with densities of louse nauplii and L. salmonis copepodids in the water at time lags of 0 and 1 wk, and 1 and 2 wk, respectively. This survey demonstrated a reduction in densities of L. salmonis larvae in the plankton (an indication of L. salmonis infectious pressure) between the 2 cycles and indicated that the farm was an important source of L. salmonis larvae. The application of anti-louse treatments using emamectin benzoate reduced the numbers of gravid L. salmonis at the farm, and this was the main factor influencing the apparent reduction in L. salmonis infectious pressure in the loch between cycles.     

Exposure of juvenile chinook and chum salmon to chemical contaminants in the Hylebos Waterway of Commencement Bay, Tacoma, Washington

The Hylebos Waterway is an industrialized waterway ofCommencement Bay, Tacoma, Washington, that is severelycontaminated with aromatic and chlorinatedhydrocarbons in the sediment. Juvenile chinook (Oncorhynchus keta) and chum salmon (O.tshawytscha) inhabit this waterway for a few days orweeks during their outmigration from freshwaterstreams to saltwater. The purpose of thisinvestigation was to determine to what degree juvenilechum and chinook salmon captured from the HylebosWaterway might bioaccumulate organic contaminants. These levels of exposure will be compared to previousstudies where such exposures have been linked tobiological dysfunction in juvenile salmon. Theresults showed that juvenile chum and chinook salmonfrom the Hylebos Waterway take up a wide range ofchemical contaminants, compared to fish fromhatcheries or reference estuaries. These contaminantsinclude high and low molecular weight polycyclicaromatic hydrocarbons (PAHs), polychlorinatedbiphenyls (PCBs, including the toxic congeners 105 and118), hexachlorobutadiene (HCBD), hexachlorobenzene(HCB), DDTs, heptachlor, and several pesticides. Immunohistochemical examination of the gill and gut injuvenile chum salmon from the Hylebos Waterway showedthe induction of the P450 metabolizing enzyme. Moreover, concentrations of contaminants in juvenilechinook and chum salmon from the Hylebos Waterway arecomparable to levels previously shown to be associatedwith biological injury in juvenile chinook salmon,such as impaired growth, suppression of immunefunction as demonstrated by reduced B cell function,and increased mortality following pathogen exposure.     

Isolation, primary structure, and biological and immunological properties of pink and chum salmon insulins

1. Insulins have been isolated from islet tissue of pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon. The primary structure of chum and pink salmon insulins was found to be identical. Compared to the amino acid sequence of human insulin, the salmon insulins under study differed at 14 positions. 2. Biological activity of pink salmon insulin was 83% of that of standard porcine insulin. 3. The immunological properties of fish insulins were investigated in specific radioimmunoassay (RIA) systems, based on porcine and pink salmon insulins. 4. A significant difference in the antigenic determinants of these fish and mammalian hormones was revealed.     

Comparative anatomy of the dorsal hump in mature Pacific salmon

Mature male Pacific salmon (Genus Oncorhynchus ) demonstrate prominent morphological changes, such as the development of a dorsal hump. The degree of dorsal hump formation depends on the species in Pacific salmon. It is generally accepted that mature males of sockeye (O. nerka ) and pink (O. gorbuscha ) salmon develop most pronounced dorsal humps. The internal structure of the dorsal hump in pink salmon has been confirmed in detail. In this study, the dorsal hump morphologies were analyzed in four Pacific salmon species inhabiting Japan, masu (O. masou ), sockeye, chum (O. keta ), and pink salmon. The internal structure of the dorsal humps also depended on the species; sockeye and pink salmon showed conspicuous development of connective tissue and growth of bone tissues in the dorsal tissues. Masu and chum salmon exhibited less‐pronounced increases in connective tissues and bone growth. Hyaluronic acid was clearly detected in dorsal hump connective tissue by histochemistry, except for in masu salmon. The lipid content in dorsal hump connective tissue was richer in masu and chum salmon than in sockeye and pink salmon. These results revealed that the patterns of dorsal hump formation differed among species, and especially sockeye and pink salmon develop pronounced dorsal humps through both increases in the amount of connective tissue and the growth of bone tissues. In contrast, masu and chum salmon develop their dorsal humps by the growth of bone tissues, rather than the development of connective tissue.     

Vector potential of the salmon louse Lepeophtheirus salmonis in the transmission of infectious haematopoietic necrosis virus (IHNV)

To better understand the role of vector transmission of aquatic viruses, we established an in vivo virus-parasite challenge specifically to address (1) whether Lepeophtheirus salmonis can acquire infectious haematopoietic necrosis virus (IHNV) after water bath exposure or via parasitizing infected Atlantic salmon Salmo salar and if so, define the duration of this association and (2) whether L. salmonis can transmit IHNV to naive Atlantic salmon and whether this transmission requires attachment to the host. Salmon lice which were water bath-exposed to 1 x 10(5) plaque-forming units (pfu) ml(-1) of IHNV for 1 h acquired the virus (2.1 x 10(4) pfu g(-1)) and remained IHNV-positive for 24 h post exposure. After parasitizing IHNV-infected hosts (viral titer in fish mucus 3.3 x 10(4) pfu ml(-1)) salmon lice acquired IHNV (3.4 x 10(3) pfu g(-1)) and remained virus-positive for 12 h. IHNV-positive salmon lice generated through water bath exposure or after parasitizing infected Atlantic salmon successfully transmitted IHNV, resulting in 76.5 and 86.6% of the exposed Atlantic salmon testing positive for IHNV, respectively. In a second experiment, only salmon lice that became IHNV-positive through water bath exposure transmitted IHNV to 20% of the naive fish, and no virus was transmitted when IHNV-infected salmon lice were cohabitated but restrained from attaching to naive fish. Under laboratory conditions, adult L. salmonis can acquire IHNV and transmit it to naive Atlantic salmon through parasitism. However, the ephemeral association of IHNV with L. salmonis indicates that the salmon louse act as a mechanical rather than a biological vector or reservoir.     

Identification of the sex chromosome pair in chum salmon (Oncorhynchus keta) and pink salmon (Oncorhynchus gorbuscha)

Fluorescence in situ hybridization (FISH) using a probe to the male-specific GH-Y (growth hormone pseudogene) was used to identify the Y chromosome in the karyotypes of chum salmon (Oncorhynchus keta) and pink salmon (Oncorhynchus gorbuscha). The sex chromosome pair is a small acrocentric chromosome pair in chum salmon and the smallest metacentric chromosome pair in pink salmon. Both of these chromosome pairs are morphologically different from the sex chromosome pairs in chinook salmon (Oncorhynchus tshawytscha) and coho salmon (Oncorhynchus kisutch). The 5S rRNA genes are on multiple chromosome pairs including the sex chromosome pair in chum salmon, but at the centromeres of two autosomal metacentric pairs in pink salmon. The sex chromosome pairs and the chromosomal locations of the 5S rDNA appear to be different in all five of the North American Pacific salmon species and rainbow trout. The implications of these results for evolution of sex chromosomes in salmonids are discussed.     

Preferences of invasive Ponto-Caspian and native European gammarids for zebra mussel (<Emphasis Type="Italic">Dreissena polymorpha</Emphasis>, Bivalvia) shell habitat

The migratory behavior and swimming patterns of anadromous upstream migratory fish have been poorly described in the Shibetsu River in eastern Hokkaido, Japan. In this 2004 study, we used electromyogram (EMG) transmitters and depth/ temperature (DT) loggers to compare the upstream migratory behavior of adult male chum salmon (Oncorhynchus keta) and pink salmon (O. gorbuscha) in the canalized and reconstructed segments of the Shibetsu River, where a part of canalized section was preliminary reconstructed meander to restore a more natural section. The EMG transmitter and DT logger were externally attached to the left side of the body, below the front edge of the dorsal fin. Fish of both species often migrated along the riverbanks and near the bottom of the water column, sometimes engaged in holding behavior, which was defined as cessation of swimming during their upstream migration for 5 minutes. Modal swimming depth calculated by DT loggers for chum salmon (0.2–0.4 m) was shallower than pink salmon (0.6–0.8 m). Further, modal swimming speeds measured by calibrated EMG for chum salmon (0.2–0.4 BL s⁻¹) were slower than pink salmon (1.2–1.4 BL s⁻¹). Pink salmon swam faster as well as in relatively deeper than chum salmon, suggesting that they expend more energy than chum salmon in the reconstructed segment. Based on these results, it seemed likely that the upstream migration behavior of chum and pink salmon was different with species-specific strategies.     

Lacustrine growth of juvenile pink salmon and a comparison with sympatric sockeye salmon

The growth rates of naturally sympatric juvenile pink Oncorhynchus gorbuscha and sockeye Oncorhynchus nerka salmon were compared in a common lacustrine environment in south‐west Alsaka, an unusual opportunity given the normal disparity in freshwater residence time of these two species. Fork length ( L _F) frequency distributions of juvenile pink salmon caught in the lake during the summer in 1991 and 1999–2003 indicated a growth rate of 0·54 mm day⁻¹, 54% greater than the estimated growth rate of juvenile sockeye salmon sampled from 1958 to 2003 (0·35 mm day⁻¹). Examination of daily growth rings on otoliths indicated that pink salmon in Lake Aleknagik grew an average of 1·34 mm day⁻¹ in 2003 but sockeye salmon grew only 0·63 mm day⁻¹(average specific growth rates were 3·0 and 1·8% day⁻¹, respectively). Pink salmon increased from c . 32 mm L _F and 0·2 g at emergence to 78 mm L _F and 3·0 g within 3–4 weeks. After experiencing these rapid growth rates, the pink salmon appeared to leave the lake by late July in most years. The diets of pink and sockeye salmon in the littoral zone of the lake were very similar; >80% of the stomach contents consisted of adult and pupal insects and the remainder was zooplankton. This high degree of diet overlap suggested that the observed differences in growth rate were not attributable to variation in prey composition.     

Characterization of proteases in the skin mucus of Atlantic salmon (Salmo salar) infected with the salmon louse (Lepeophtheirus salmonis) and in whole-body louse homogenate

As part of an investigation of the biochemical interactions between the salmon louse Lepeophtheirus salmonis and Atlantic salmon Salmo salar, we characterized protease activity in the skin mucus of noninfected Atlantic salmon and Atlantic salmon infected with L. salmonis and in an L. salmonis whole-body homogenate. Zymography revealed that mucus from infected salmon contained a series of low-molecular-mass (17-22 kDa) serine proteases that were not present in the mucus of noninfected salmon. Based on molecular mass, inhibition studies, and affinity chromatography, the series of proteases was identified as being trypsin-like. Similar proteases were observed in the L. salmonis homogenate and in mucus from noninfected Atlantic salmon following a 1-hr incubation with live L. salmonis. An antibody raised against Atlantic salmon trypsin failed to recognize any proteases in the mucus of noninfected salmon or infected salmon or in the L. salmonis homogenate. Collectively, these findings suggest that the trypsin-like proteases present in the mucus of infected Atlantic salmon were produced by L. salmonis, possibly to aid in feeding and evasion of host immune responses.     

Wild and hatchery reproduction of pink and chum salmon and their catches in the Sakhalin-Kuril region,Russia

In the Sakhalin-Kuril region hatchery culture of pink and chum salmon is of great importance compared to other regions of the Russian Far East. During the last 30 years the number of hatcheries increased two-fold, and significant advances were made in hatchery technologies. As a result, chum salmon capture in regions where hatcheries operate (southwestern and eastern Sakhalin coasts, and Iturup Island) was 9 times as high during 2006–2010 than during 1986–1990, whereas wild chum salmon harvest markedly declined. Recent dynamics in pink salmon catch appear to track trends in natural spawning in monitored index rivers, suggesting natural-origin pink salmon play a dominant role in supporting the commercial fishery. It remains uncertain as to whether hatcheries have substantially supplemented commercial catch of pink salmon in this region, and I recommend continued research (including implementing mass marking and recovery programs) before decisions are made regarding increasing pink salmon hatchery production. Location of hatcheries in spawning river basins poses problems for structuring a management system that treats hatchery and wild populations separately. Debate continues regarding the existence and importance of density-dependent processes operating in the ocean environment and the role hatcheries play in these processes. Loss of critical spawning habitat for chum salmon in the Sakhalin-Kuril region has lead to significant declines in their abundance. I conclude by recommending increases in releases of hatchery chum salmon numbers in the region to help recover depressed wild populations and provide greater commercial fishing benefits in the region.