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Catching smolt: Crew spread purse seine net.
The BC Pacific Salmon Forum is an independent citizen body using science and stakeholder dialogue to advance sustainable governance of BC Pacific salmon. Three objectives were identified for a two year research program focused primarily in the Broughton Archipelago: 1) To determine whether salmon farms in the Broughton impact sea lice loads on wild salmon and if so how; 2) To determine whether the survival of individual wild fish is compromised due to increased lice loads; and 3) To determine if any reduced survival of individual salmon has consequences on salmon populations, and if so, are there management techniques that can be put in place to mitigate any risk to wild salmon?
In 2008 the Forum provided funding to 15 collaborative research projects that involved more than 35 scientists. A wealth of new knowledge was gained in areas ranging from fish health and physiology, to oceanography, host-parasite interactions and the biology of sea lice. While the research focused predominantly on the Broughton Archipelago region, projects were also undertaken in other regions along the BC coast, some still ongoing. The findings from the 2008 research period built upon those from earlier seasons and will assist scientists in reaching a clearer understanding of ecosystem interactions in the Broughton and potentially beyond. The following provides a brief overview of the preliminary findings from the 2008 field season.
Adding smolt to water column.
Aquatic organisms are in intimate contact with the ecosystems in which they reside and scientists are only just beginning to scratch the surface of the complex interactions between fish, parasites and pathogens, and the environments in which they coexist. The environment plays a defining factor in the outcomes of those interactions and a clearer understanding of the mechanisms of transport and exchange within the aquatic environment is vital to understanding the dynamics of salmon populations in our waters.
Oceanographic circulation and sea lice dispersion models have been developed for the Broughton Archipelago and have been utilized to simulate oceanographic conditions and lice concentrations for defined collection periods(1). Ocean currents, salinities and temperatures in the model simulations were found to agree relatively well with data collected from the field leading to the expectation that these models may prove to be useful tools for investigating wild/farmed salmon interactions with sea lice and examining farm management strategies in the future.
One of the difficulties in understanding salmon dynamics is a lack of knowledge regarding the exact origins of a fish. Understanding the origins of each sample assists scientists in determining migration patterns and in identifying potential significant differences in infection patterns of different stocks. It is well known that different stocks of organisms do not share identical genetic makeup, some stocks may possess greater or poorer resistance to pathogens and this knowledge is useful in identifying stocks that may be at greater risk of impact from infection.
Micro-chemical analyses demonstrated that it is possible to discriminate the natal origins of pink and chum salmon sampled in the Broughton Archipelago based on otolith composition(2). Juvenile salmon rearing in different stream systems are subjected to variations in water chemistry and these can be traced by micro-analysis. Salmon of known origin were correctly identified approximately 85% of the time suggesting that this research could provide a useful analytical tool in the ongoing efforts to better understand salmon migration pathways in the Broughton Archipelago.
Mike Sackville studies specimens under a microscope.
There has been a great deal of debate surrounding the dynamics of juvenile salmon and associated sea lice. While the answers still remain somewhat elusive, certain patterns are merging. Extensive sampling programs by several different research groups, over 5 or more years, have demonstrated that there are significant shifts in both salmon and lice populations, although these do not always appear to share a positive correlation. Total catches of juvenile wild pink and chum were high in 2003, but declined substantially for both species in 2004. Although total catches and catch-per-unit-effort (CPUE) remained relatively constant for both species from 2004 to 2008, the number of fish captured each year varied widely between sampling locations and between years(3). In 2008, a total of 622 sets were completed using beach seine and purse seine fishing gear with sampling occurring over 8-10 days each month, near the end of March, April, May and June. In 2008 a total of 22,995 juvenile pink salmon and 9,394 juvenile wild chum salmon were captured, very similar to the number captured in 2007.
It is generally considered that smaller fish are more susceptible to infestation by sea lice. In 2008, a large number of juvenile salmon were examined for sea lice. There were large variations in both incidence and severity of infection by L. salmonis on juvenile pink and chum salmon between years, and also between different locations within any particular year(3,4). The prevalence and abundance of L. salmonis on juvenile pink and chum salmon has continuously declined since 2004 although the explanations for this decline are not fully understood. The percent of lethally-infected pink salmon declined from approximately 1% in 2005 to zero in 2008. Lab studies have demonstrated that once fry reach approximately 0.7g they are highly resistant to the effects of lice(4).
A question that has yet to be clearly answered is ‘where do the lice come from?’. Farmed salmon are not infected when they enter the saltwater pens therefore, logically, the lice are of a natural source in the environment. However, we have yet to clearly identify that reservoir.
This catch: mostly herring, few smolt.
It is accepted that there exists a relationship between sea lice on wild salmon and salmon farms, although that relationship is, as yet, incompletely defined. Regardless, prudence demands that a cautionary approach be taken to managing the interactions. Mathematical modeling has been used to connect data from farms with that from wild salmon sampling. SLICE® has been shown to reduce lice on farms and therefore transmission to wild juvenile salmon. Reduced sea lice transmission implies improved wild salmon survival but it remains unknown if SLICE® is sufficient to conserve and restore wild salmon(6). Studies in the Broughton indicate that infections begin in the winter. In the winter of 2008, salmon farms at the Knight Inlet/Tribune Channel region were treated with SLICE® and thus were virtually free of sea lice. In the winter of 2007/2008 sticklebacks around these farms were heavily (~70%) infected with chalimus stages of C. clemensi, an infection that occurred throughout the winter and continued after SLICE® was administered. The prevalence and intensity of the infection before and after treatment indicated that C. clemensi was being continually produced from a source other than the salmon farms(5).
An important component of management is an understanding of the range and density of lice in the environment. In an attempt to quantify the abundance and map the distribution of larval lice in the region, bi-weekly plankton tows were conducted throughout the Broughton in 2007-2008(7). The abundances of naupliar and copepodid stages of both Lepeophtheirus salmonis and Caligus clemensi were low. L. salmonis were found to be most abundant near recently active fish farms while C. clemensi showed little or no spatial association with fish farms, but may have been associated with herring aggregations.
That sea lice migrate towards the surface at night and move deeper during the daylight hours has long been accepted. However, recent research suggests that this may be less clear than previously thought. When L. salmonis nauplii and copepodids were held in a 10 m column suspended in the ocean there did not appear to be a preference for any particular depth and there was no apparent effect of daytime period on their vertical distribution(8). In the absence of lice, juvenile pink salmon distribution changed slightly from day to night; fish less than 3 weeks post-saltwater entry (up to ~0.5 g) were typically found to occupy the top 3 m of the water column. When lice and fish were held together in the column, salmon dispersed more during the day, but their pattern of distribution did not change at night. No difference in the proportion of infected vs. uninfected fish was observed relative to the vertical distribution of the fish, suggesting that infective pressure is independent of daylight and depth. There have been suggestions that juvenile salmon prey on the eggs of lice and/or the lice themselves. It is unknown whether the copepodids were following the fish as potential hosts or the fish were following the copepodids as a potential food source, only that their vertical distributions were correlated within the column when they were free to interact(8).
While much of the focus in recent years has been on population effects of lice loads, there is very little available data on the effects of infection on individual fish. Experiments in 2008 involved over 10,000 juvenile pink salmon, some as small as 0.2 g, and examined in a field setting, the effects of L. salmonis on ionic balance and swimming performance of the fish at sea lice densities of 1 to 3 lice per fish. Sub-lethal effects of 1 louse per fish on ionic balance and swimming performance were dependent on the size of the fish but significant effects occurred only on fish with a body mass of <0.5 g. A higher density of lice (2 or 3 lice per fish) was required to trigger sub-lethal effects on ionic balance and swimming performance in larger fish (0.5 – 3.7g). In all cases, sub-lethal effects were only detected when the life stage of the louse was at least chalimus 3-4. Compared with the sub-lethal changes observed for <0.5 g fish with 1 louse per fish, increasing lice density up to 3 lice per fish did not have an additive sub-lethal effect for fish of any size(9). Interestingly, naturally-infected fish appeared to be less sensitive to lice loads than did those fish that were artificially infected under lab conditions. Regardless, in all physiological studies mortality was minimal (<2%) among fish infected with sea lice during tests that lasted up to 28 days. Additionally, there is evidence from more than one study that juvenile salmon may possess effective mechanisms for shedding lice(5,9).
Fish health is a complicated field and involves a great deal more than a simple host-parasite interaction. Fitness is impacted by early rearing, natural feed supplies, genetics, environmental conditions, etc. Many factors lead to the state we call disease, and a pathogen is merely one factor in a complicated equation. Merely finding a parasite or pathogen on or in any organism is not to be confused with a state of disease. If a fish is in a healthy state it will be quite capable of combating any number of challenges. When that same animal is compromised by some factor, biotic or abiotic, the scenario can play out quite differently. For this reason it is important that we gain knowledge regarding the overall health of a population of fish.
Juvenile pink salmon out-migrating through the Broughton Archipelago in 2008 were evaluated for physiologic condition, sea lice, other parasites, bacteria, viruses, and microscopic lesions. Histopathology found microscopic lesions that are considered to have developed in the saltwater environment, yet were not attributable to sea lice(10). This suggests that there are other, as yet unidentified, factors that are impacting juvenile salmon in the Broughton. In nature, parasites are a normal and natural occurrence but the question remains, are salmon unhealthy because they are infested with lice, or are they infested with lice because they are unhealthy?
It is important to highlight the fact that salmon populations have shown significant declines all along the BC coastline. The population depressions are not limited to the Broughton. However, the research funded by the Forum focused on this region and found that pink salmon production out of the Glendale Channel was approximately half of that of 2007 and this is likely attributed to low adult escapement in 2007 relative to higher numbers observed in 2006(11).
Installation of a resistivity counter on the Glendale spawning channel will allow accurate egg–to-fry survival numbers to be acquired over the coming years to assess survival trends over time. Preliminary estimates of escapement derived via over-flight assessments indicate replacement of brood in some systems and much reduced returns relative to brood in other systems. The overall trend in escapement indicates a significant decrease in overall numbers between 2006 and 2008, with the Glendale at 91% fewer numbers of returning adults than in 2006. The trends are similar to those observed in other systems outside of the Mainland Inlets (i.e., Central and North Coast) and do not appear to be a localized event as was encountered in 2002 and 2003(11).
Water column raised for inspection.
While the overall focus of the Forum-funded research was the Broughton Archipelago, there were research projects underway in other regions that were fully or partially funded by the BC Pacific Salmon Forum. Juvenile salmon in the Bella Bella region, an area that lacks salmon farms, were found to host low levels of sea lice (3.5%), which are considered to be natural background levels for the region. In regions where there are fish farms, significantly more juvenile salmon were found to host sea lice in areas near to farms compared to areas farther from farms(12). Elevated levels of lice nearer to farms included significantly greater proportions of L. salmonis, a salmon-specific species, than C. clemensi, which is more of a generalist (found on numerous fish species).
The Kitasoo Fisheries Program undertook a monitoring program and collected juvenile salmon in areas around and away from salmon farms between 2004 and 2008 and assessed them for sea lice. Both C. clemensi and L. salmonis were observed on fish in all areas sampled. In 2008, lice levels were the lowest of all years data were collected. Prevalence and intensity of L salmonis was low in all areas sampled and in all years examined for both chum and pink salmon. L. salmonis abundance levels were higher on pink salmon caught around farms in 2005 and 2006 but not in 2007 or 2008 when data were compared to the reference area(13).
The Gulf Islands area in the Strait of Georgia is a major rearing area for all species of juvenile Pacific salmon. As part of another study in 2008, researchers were able to measure sea lice levels on these juvenile salmon. The levels of infection were approximately 70% and were mostly C. clemensi. There were no salmon farms in the area, indicating that large, natural infections of sea lice can occur(5).
The purpose of the Broughton research strategy was to attempt to clarify some of the interactions between farmed and wild fish in the marine ecosystem with a primary focus on wild pink and chum salmon populations in all facets of their life cycle. The research considered what factors in the ecosystem may impact these wild fish populations, the incremental risk, if any, associated with salmon farming, and how any potential risks could be mitigated. While many questions remain unanswered and new questions have arisen as a result of a rigorous collection of scientific investigations, the research findings expand our existing knowledge and will continue to provide a foundation for future studies that will shed more light on the inter-relationships among all factors.
Note: This article is based on individual research summaries provided to the BC Pacific Salmon Forum and the information has not been subjected to a formal peer review process. The full summaries of the research projects highlighted within the above may be found at the BC Pacific Salmon Forum’s website at www.pacificsalmonforum.ca
Researchers involved in the BC Pacific Salmon Forum-funded projects:
Duration: ’06 – ‘09
Funded by: BC Pacific Salmon Forum
For information contact: firstname.lastname@example.org