Recovery potential of black rockfish, Sebastes melanops Girard, recompressed following barotrauma

Effects of Gill-Net Trauma, Barotrauma, and Deep Release on Postrelease Mortality of Lake Trout

Unaccounted postrelease mortality violates assumptions of many fisheries studies, thereby biasing parameter estimates and reducing efficiency. We evaluated effects of gill-net trauma, barotrauma, and deep-release treatment on postrelease mortality of lake trout Salvelinus namaycush. Lake trout were captured at depths up to 65 m with gill nets in Priest Lake, Idaho, and held in a large enclosure for 10–12 d. Postrelease mortality was the same for surface-release–and deep-release–treated fish (41%). Mixed-effects logistic regression models were used to evaluate effects of intrinsic and environmental factors on the probability of mortality. Presence of gill-net trauma and degree of barotrauma were associated with increased probability of postrelease mortality. Smaller fish were also more likely to suffer postrelease mortality. On average, deep-release treatment did not reduce postrelease mortality, but effectiveness of treatment increased with fish length. Of the environmental factors evaluated, only elapsed time between lifting the first and last anchors of a gill-net gang (i.e., lift time) was significantly related to postrelease mortality. Longer lift times, which may allow ascending lake trout to acclimate to depressurization, were associated with lower postrelease mortality rates. Our study suggests that postrelease mortality may be higher than previously assumed for lake trout because mortality continues after 48 h. In future studies, postrelease mortality could be reduced by increasing gill-net lift times and increasing mesh size used to increase length of fish captured.

Clinical signs of barotrauma in golden perch, Macquaria ambigua (Richardson), and associated effects on post-release mortality and health

This study assessed the effects of different retrieval depths (2, 10 or 20 m), surface intervals (none or 15 min) and release methods (untreated, vented or recompressed) on the incidence of external and internal clinical signs of barotrauma (ECSB and ICSB) and post-release mortality in golden perch, Macquaria ambigua (Richardson). Fish were assessed for ECSB before and after surface intervals and either monitored for mortality over 3 days in two deep cages or killed for internal examination. When all fish were left untreated, short-term mortality increased with retrieval depth from 0% and 4.2% among 2 and 10-m fish, respectively, to 19.2% among 20-m fish; while surface interval only affected the incidence of two ECSB (excess buoyancy and a prolapsed cloaca). Mortality was also greater among 20-m fish that were subjected to a 15-min surface interval and left untreated (22.2%) or vented (22.2%) than those that were recompressed (5.6%). Of the ECSB, only exophthalmia was associated with increased mortality, with half of the affected fish dying. However, many fish retrieved from 10 and 20 m also sustained numerous ICSB, including compressed gonads or vital organs and ruptured or collapsed, haemorrhaging swimbladders that remained deflated for up to 3 days after release.

Identification of biomarkers indicative of barotrauma and recovery in black rockfish Sebastes melanops

A Sebastes-specific complementary DNA (cDNA) microarray was developed to identify potential biomarkers involved in the capture stress and recovery of Sebastes species if they are assisted in returning to their original depth of capture following barotrauma. Black rockfish Sebastes melanops were exposed to simulated decompression from 450 kPa (c. 35 m depth) (which resulted in barotrauma) and subsequent recompression. Sebastes melanops were sampled for liver tissue at days 3, 15 and 31 post-barotrauma. Potential candidate genes were identified from the microarray and then quantitative real-time PCR (qrt-PCR) was used to validate expression levels in biological replicates. Six potential biomarkers associated with the innate immune system were identified that were up-regulated in liver tissue at 3 days post-barotrauma: complement C1q-like protein 2, complement component C3, complement regulatory plasma protein, serum amyloid A-5, c-type lysozyme and hepcidin precursor type I. In addition, complement c1q was correlated to the presence of a ruptured swimbladder, providing further support that this gene may be a good biomarker of injury and recovery. Immune genes were no longer up-regulated at day 31 post-barotrauma, a good indication of recovery in S. melanops.