Steller Sea Lion Response to Prespawning Eulachon in Southeastern Alaska
Predictable, seasonally abundant aggregations of energy-rich forage fish may be critical resources for the survival and reproductive success of Steller sea lions (Eumetopias jubatus). In a spring 2006 study, we estimated the energy available to Steller sea lions from prespawning aggregations of eulachon (Thaleichthys pacificus) in Lynn Canal and Stephens Passage, southeastern Alaska. The study was a cooperative research project involving three ABL programs: Habitat, Groundfish Assessment, and the Ocean Carrying Capacity Program’s Stock Identification Laboratory. Funding was also provided by the AFSC’s National Marine Mammal Laboratory.
Eulachon are high-lipid, energy-rich fish that spawn in the spring and are likely vital energy sources for sea lions at a time of high energetic demands associated with pregnancy, lactation, and fasting. Our objectives were to characterize five spawning runs of eulachon by: 1) estimating the biomass of prespawning aggregations of eulachon available to marine predators prior to the eulachon’s upriver spawning migration, 2) measuring the energy content of eulachon and calculating the total energy represented by the aggregations, 3) estimating the numerical response of Steller sea lions and other marine mammals to the eulachon runs, 4) characterizing the population structure of the five runs using genetics, and 5) characterizing eulachon movement within the marine environment prior to upriver migration for spawning.
Surveys were conducted in Taku, Chilkat, Lutak, and Taiya inlets, and in Berners Bay over the period of 2 April to 6 May 2006. Hydroacoustic surveys were performed to estimate fish biomass and behavior in the marine environment. Midwater trawls and gillnets were used to sample fish to delineate acoustic data. These methods of capture as well as dipnets and fish traps were used to collect fish specimens for morphological measurements, nutritional analyses, and genetic analyses. Aerial surveys were used to enumerate sea lions attending the eulachon runs and at adjacent sea lion haulouts. In addition, aerial surveys provided direction to help determine locations and timing of the hydroacoustic trawl and gillnet surveys.
Preliminary data analysis appears to show several interesting aspects of sea lion and eulachon behavior. The Berners Bay run was very low this year, and sea lions did not show up to feed as they have in the past several years. We are currently unaware whether this has ever happened previously. Inlets such as Taku and Chilkat are large river systems with large potential eulachon runs. However, due to the marine and freshwater bathymetry and fish behavior, sea lions are not able to efficiently feed on the eulachon. We recorded substantial numbers of eulachon in these two locations but did not document many sea lions feeding, nor have there been many observed in past years. The fish appear to move quickly from marine to fresh water, not schooling much if at all in saltwater, and then spread out in the large riparian system to spawn.
We also documented a vast osmerid larval layer (capelin and eulachon) throughout Berners Bay and Taku Inlet and possibly in the other three inlets, but we cannot verify species identification in the other inlets due to lack of trawling. Lastly, our acoustic surveys seem to show fish density peaking a day or two before the sea lion feeding numbers peak. For example, when the fish were most dense in Lutak Inlet, sea lion numbers were about 300. Two days later, fish density was much less, but sea lion numbers had increased to about 500.
By David Csepp and Johanna Vollenweider
Juvenile Rockfish Habitat Utilization
New research detailing the relationship between juvenile rockfish Sebastes spp. and benthic habitat is under way at ABL’s Little Port Walter (LPW) Marine Station. The relative degree in which juvenile rockfish utilize different habitat features with or without the presence of predators or under varied light levels is mostly unknown. The results of this research will assess the strength of association between juvenile rockfish and several habitat features, including emergent epifauna such as sponge and coral as well as cobble and gravel. These assessments will be helpful in determining the relative productivity of sponge and coral and will aid in establishing priorities for protecting these vulnerable habitats specifically addressed in essential fish habitat (EFH) and Habitat Areas of Particular Concern (HAPC) priorities.
Beginning in summer 2006, juvenile quillback rockfish (S. maliger) and a predator, great sculpin (Myoxocephalus polyacanthocephalus), will be captured with beach seines and dip nets and transported to the LPW Behavior Laboratory. Once fish are acclimated, two separate experiments will occur. The first experiment will identify habitat preference of juvenile quillbacks. Rockfish will be placed in rectangular aquariums with two distinct habitat types, one on each half of the aquarium. After an acclimation period, the rockfish will be periodically observed to determine which habitat types are most utilized. Replicate comparisons will be made of the six possible combinations of habitat types under daylight and night conditions.
A second experiment will determine which habitat features are utilized by juvenile quillbacks in the presence of a predator and the relative protection the habitat features provide under predatory pressure. In this suite of experiments, juvenile quillbacks and two great sculpin will be placed in aquariums with uniform habitat features throughout. Periodic observations of rockfish will determine habitat utilization and counts of the number of rockfish consumed will determine the rate of habitat-mediated predation within each habitat type. Each trial will begin with rockfish and predators separated by a clear partition. Observations will occur under both daylight and night conditions.
This project is a continuation and expansion of a pilot study originated in 2005. The pilot study provided opportunities to develop experimental design, refine observational arrays, obtain suitable habitats, perform preliminary behavioral trials, and perfect the logistics of capturing and holding juvenile rockfish. In 2005, 350 juvenile quillback rockfish with an average length of 22 mm were captured and held in captivity for more than 6 months. Preliminary results indicated that juvenile quillbacks were about three times more likely to be observed in the coral habitat than in any other habitat type, and gravel was the least preferred habitat type. The lowest rate of predation was observed in aquariums with coral, whereas the highest rate of predation occurred in aquariums with gravel. The average number of rockfish consumed per trial was about four times higher in the gravel habitats than in the coral habitats.
