Marine survival of chum salmon that return to Hidden Falls Hatchery have exhibited a declining trend since the early 1990’s and early predation by rockfish is hypothesized to be the primary cause. In spring 2021, NSRAA took first steps to address the decline by identifying the most prevalent rockfish species and their present and past consumption of hatchery fry relative to other prey at sites where hatchery fish are released. The most numerous rockfish were Black, Dusky, Quilback, and Silvergray rockfish in Kasnyku Bay where the largest chum releases occurred (see tables below) and these same species were most common at the release site near Whitewater Bay, except sculpin replaced Silvergray rockfish in being one of the highest numbered predators.
Catch rates using hook-and-line for the most common species did not significantly change before and during the time when 2g chum fry were released at both sites (see figures below). No change in catch rate was detected near Whitewater Bay when 4g chum fry were released at this site either. However, a significant increase in rockfish catch occurred at Kasnyku Bay during the 4g chum fry release in June. These results match observations that included no visual evidence of predation except when 4g chum fry were released at Kasnyku Bay when rockfish were observed feeding in large schools on chum fry at the water surface. Initial results of stomach contents obtained from captured predators the Alaska Department of Fish and Game permitted for sampling corroborate these findings, as chum fry were in around half the stomachs from rockfish captured on days surface feeding was observed.
Next steps are to conduct stable isotope analyses on tissues sampled from the permitted rockfish species. The turnover rate of stable isotopes in tissues of predators varies between species, fish age and size, and body part sampled, but is generally slow. Therefore, isotope analysis can be used to identify predator diets before capture for comparison to stomach contents at capture to determine if the hatchery releases cause predator shifts in diet. Isotope analysis can tell us this because hatchery fish are raised on fish food that has a unique carbon signature compared to prey in the marine environment, and the difference can be detected in the tissues (liver, muscle, fin, scales), and eye lenses sampled from the captured rockfish. The tissues inform the short term (days to months) prey items and the eye lenses can tell us how often hatchery fish were in the predator’s diet since the predator’s birth. This is possible because the eye lens grows from a core at birth by forming layers with time like an onion. The layers are delaminated and dried and isotope analysis performed as done with tissues to determine if hatchery fish were consumed. The fish whose eye lenses were extracted will also be aged by removing the otolith and counting annuli on it. The otolith nuclei indicates age zero when the lens core is created and the outer lens and annuli represent the fish diet and age at death, so the number of shifts in diet in the predators life can be determined from birth to time of capture.
The proportion of hatchery fish in the predator’s diet controls the carbon signature in the predator’s flesh. The stronger the hatchery carbon signature, the higher the consumption of hatchery fish. However, one factor is that upon release, hatchery fish no longer feed on hatchery food but capitalize on natural prey in marine waters and so gradually lose their unique carbon signature. Therefore, around 1,000 chum fry from each release group were held in an anchored net pen to feed on natural prey drifting into their enclosure. We culled 25 fish for isotope analysis the day before the fish were taken off their hatchery diet and an additional 25 per week were culled for two months to quantify rates the fish’s carbon signature becomes indistinguishable from non-hatchery prey. With this information and the isotope analyses from tissues and eye lenses, we can estimate durations and timings in the captured predators lives that hatchery fish were distinguishable and represented different proportions of their diet.
Results of the stable isotope analyses and study findings will be posted as they are developed.
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