Gillnet Capture Research Articles

Detailed Insight into Gillnet Catches: Fish Directivity and Micro Distribution

Gillnets are widely used in research and commercial fishery activities. As passive gear, gillnets can be selective and dependent on the diel migration of fish. In areas with limited littoral extent, inshore–offshore migration may cause bias in the gillnet catch. Our hypothesis was that some factors, such as gillnet saturation, fish depletion, or chemical cues, could be the cause of the bias. We used a total of 66 CEN gillnets deployed at Římov Reservoir parallel to the shore at different positions of littoral-pelagic gradient. Individual fish direction was recorded from inshore, offshore, or unknown direction (i.e., entangled fish). A total of 5791 fishes from nine different species were caught. For most fish, it was possible to determine their directivity, and most fish were captured in littoral or first pelagic gillnets. Shallower and deeper benthic gillnets differed in their bleak (Alburnus alburnus) catch. No significant differences were found between fish directions. At the species level, only asp (Leuciscus aspius) and ruffe (Gymnocephalus cernua) showed differences between the captured directions in one case. The results support the assumption that gillnet capture is a random process that to a great extent is connected to random local movements. This is good news for fish monitoring projects. Sampling catch is likely to reflect true changes in the fish community, and not the effects of the deployment of the sampling gear. The experiment also showed that fish directivity statistics can be used for investigation of fish behavior and gear performance.

Post‐capture recovery is mediated by shifts in allostatic and metabolic dynamics in a threatened and endemic rhino ray

AbstractUnderstanding how animals respond to stressors is critical for effective conservation. The present study aimed to evaluate the physiological recovery of the Shortnose Guitarfish, Zapteryx brevirostris after being incidentally captured by artisanal fleets and compared it to guitarfish with delayed mortality. Moreover, the study aimed to test the efficiency of a rehabilitation protocol for the species. To this end, individuals landed alive were transported to the rehabilitation facilities and kept in tanks under monitoring for 3 days. Physiological markers indicative of allostatic overload (i.e., lactate and phosphorus) and energy metabolism (i.e., glucose and β‐hydroxybutyrate) were assessed in the serum of the studied guitarfish: right after gillnet capture (i.e., post‐capture) and on rehabilitation (i.e., pre‐monitoring, post‐monitoring, as well as guitarfish that deceased during monitoring – post‐mortem). Results showed that the transportation and additional handling employed in the rehabilitation protocol did not increase the stress response, pointing to its potential efficacy in recovering debilitated, incidentally caught guitarfish before release. As for recovery success of monitored guitarfish, lactic acidosis was fully reverted after a 3‐day rest period, as was allostatic overload. The same was not observed in recovery failure. Correlation tests revealed key relationships between physiological markers, shedding light on how physiological pathways help guitarfish cope with capture and handling stress. This study highlights the valuable application of conservation physiology principles to fisheries management of this endemic and threatened guitarfish species.

The effect of fishing-capture stress on the oxygen uptake rate and swimming activity of the holocephalan Callorhinchus milii.

Overfishing, capture mortality, and consequences following the release of surviving animals represent severe threats to chondrichthyans. Although holocephalans are common bycaught and discarded species, other than postrelease mortality, little is known of fishing capture stress impacts. The stress response elicited after capture, essential to increase survival chances, is energetically demanding and affects the amount of energy available for other biological activities, with potential long-term impairments. We measured the effect of 30-min simulated gillnet capture on oxygen uptake rate (ṀO2 ), a proxy for metabolic rate and energy use, on recovery pattern, and on swimming activity of elephant fish (Callorhinchus milii). Immediately after simulated capture, Active and Inactive ṀO2 , measured during swimming and resting periods, respectively, were 27.5% and 43.1% lower than precapture values. This metabolic decline is likely an adaptation for reducing the energy allocated to non-essential activities, thus preserving it to sustain the stress response and processes essential for immediate survival. Supporting this, after gillnet capture, animals decreased their swimming time by 26.6%, probably due to a reduction in the energy allocated to movement. After 7 days, swimming activity and both Inactive ṀO2 and Active ṀO2 returned to precapture values. Although metabolic decline may enhance survival chances, the associated decreased swimming activity might increase predation risk and slow the physiological recovery after a fishing event. Moreover, some of the activities involved in Inactive ṀO2 are fundamental for life maintenance and therefore its depression after a capture event might have long-term repercussions for life sustenance and health.

KOMPOSISI HASIL TANGKAPAN IKAN DAN TINGKAT KERAMAHAN LINGKUNGAN ALAT TANGKAP JARING INSANG DI KUALLO SOKKAM, SUMATERA UTARA

The development of fishing technology emphasized environmentally friendly technology in hopes of utilizing a sustainable fishery resource and knowing the type and composition of fish catches. The purpose of this research is to know the composition of fish catches and the level of environmental friendliness of the net capture equipment based on the FAO (1995). The method used in this study was a survey method with 2 objects of the Fish capture equipment unit. The composition of fish catches by type consists of 8 species. The species dominated by the type of fish is the female bloating (Rastrellinger brachysoma) amounting to 34%. The value of environmental friendliness of the gill net capture is 26. Based on the number of values categorized as a friendly capture tool that is a friendly environment.

Comparison of fish communities using environmental DNA metabarcoding and capture methods in a freshwater lake: A new set of universal PCR primers

Freshwater biodiversity is under pressure from the detrimental effects of climate change, habitat degradation, biological invasion, and overfishing. Environmental DNA (eDNA) obtained directly from environmental samples can be used to evaluate the distribution of aquatic species. We developed a new set of universal PCR primers (16 S 200) for eDNA metabarcoding from mitochondrial DNA sequences. Using the new 16 S 200 primers, we detected the presence of 27 fish species distributed across four families in Lake Gehu, in southeastern China. eDNA metabarcoding and live capture methods identified 16 species in common; however, the two methods identified nine and 12 unique species, respectively. Data from eDNA metabarcoding were more consistent with results from gill net capture methods than from ground cage capture. The present study supports the effectiveness of eDNA for rapidly assessing the species composition of fish communities.This is the first attempt to designed new sets of primers for eDNA-based metabarcoding analysis of freshwater fish in China. Despite pitfalls and limitations, eDNA metabarcoding is a promising approach for assessing fish communities in freshwater lakes. Current applications of eDNA are widespread, but the new technology requires further refinement.The eDNA metabarcoding is an efficient and cost-effective method that can be used in conjunction with traditional survey methods for analyzing fish communities.

The effect of gillnet capture on the metabolic rate of two shark species with contrasting lifestyles

In this study we investigated the metabolic consequences of simulated gillnet capture in species with contrasting life-history characteristics, the gummy shark, Mustelus antarcticus and the Port Jackson shark, Heterdontus portusjacksoni. Pre-stress standard and routine metabolic rates (MR) of individuals of these species were measured and compared to the MR obtained after simulated gillnet capture. Standard MRs of M. antarcticus and H. portusjacksoni were 168.5 ± 61.9 mgO2 kg-0.67 h−1 and 144.1 ± 20.4 mgO2 kg-0.67 h−1, respectively. Routine MRs of M. antarcticus and H. portusjacksoni were 202.6 ± 63.5 mgO2 kg-0.67 h−1 and 166.4 ± 11.2 mgO2 kg-0.67 h−1, respectively. The simulated gillnet-capture treatment decreased the MR of M. antarcticus by 59.3% and increased that of H. portusjacksoni by 21.6%. Our results confirm that M. antarcticus is highly sensitive to gillnet capture, exhibiting significant variability in MR among individuals and also a high delayed mortality rate. We interpret the decrease in MR as a compensatory mechanism for reducing oxygen consumption to cope with the stress imposed by capture. Mortality in this species was associated with higher pre-stress MR, and lower post-stress MR. In contrast, H. portusjacksoni showed mild increases in MR. Their resilience to capture stress most likely relies on effective buccal pumping and lower basal MR. A mixed-effects model approach permitted us to identify the main sources of variation in the MR measured, which were the individual differences, treatment effects, species variations and delayed mortality. The comparative approach employed in this work allowed us to understand and provide reliable estimates of the effects of fishing on the MRs of these two species.

Location-specific consequences of beach seine and gillnet capture on upriver-migrating sockeye salmon migration behavior and fate

Fish released after capture, or fish interacting with gear but escaping, sometimes experience fishing-related incidental mortality (FRIM). For adult Pacific salmon migrations, knowing the magnitude of FRIM is important to estimate escapement accurately and to understand the total impact of a specific fishery. To determine how multiple gear types are associated with FRIM at different levels of maturity, we captured sockeye salmon (Oncorhynchus nerka) by both gill net and beach seine at three locations along their migration route (10%, 26%, and 72% of a 500 km freshwater migration) and determined their migratory success using biotelemetry. FRIM was higher for fish captured by gill net except at the location closest to spawning grounds. In addition, salmon captured by gill net at the lower river locations temporarily delayed migration, potentially indicating a requirement for lengthier recovery time compared with beach-seined fish. These results provide the first empirical and parallel comparison of these two common in-river fishing methods for salmon, revealing clear differences in FRIM between the two fishing methods in lower river fisheries and the importance of maturity.

Fisheries capture and infectious agents are associated with travel rate and survival of Chinook salmon during spawning migration

Following interactions with fisheries gear, fishes may experience delayed mortality or display modified behavior. The physiological status of fish at the time of capture, including the presence of infectious agents, can also influence survival outcomes. To explore the relationship between capture, infectious agents and fate, we simulated gillnet capture on adult salmon returning to a hatchery, used quantitative PCR to quantify infectious agents in non-lethal gill biopsies, and determined longevity, migratory fate, and migration rate using radio telemetry. An accompanying holding experiment investigated the relationship between infectious agents and longevity. Gillnetted fish took longer to migrate than biopsy-only fish (median 3.8 versus 2.5 days), but there was no difference in survival or migratory success among treatment groups. Longevity for fish infected with a parasite, Cryptobia salmositica, was similar between the holding (median = 7.0 days) and telemetry (7.4 days) experiments and significantly lower than that of uninfected fish (17.4 days in telemetry experiment). Males were 5.1 times more likely to arrive at spawning grounds compared to females and also migrated faster. The impact of C. salmositica on adult Chinook salmon in our study was demonstrated at the molecular (genes), physiological (plasma variables), and organism (migratory success) levels. These results demonstrate an instance where sex and infection were better predictors of migratory fate than an experimentally applied capture experience.

KOLABORASI FISH-NET DAN TECHNOLOGY UNTUK OPTIMALISASI ALAT TANGKAP IKAN

AbstractKelurahan Untia is one of the areas occupied by the fishermen and also the majority of the population work as fishermen catch fishermen. The dominant fishing gear used by fishermen in the village of Untia is surface gill net. But the use of surface gill net capture device is considered not effective and efficient because it has constraints on the process of checking the net, the old fishing process, the net tends to disappear, and the catch is considered less. So from that problem, we encourage us to create a technological innovation called FiNe-Tech (Fish Net Technology). FiNe-Tech is designed to be able to monitor fish trapped in surface gill net captures, simplify the process of catching fish in the sea, tracking the position of the jarring, speeding up the fishing process, and increasing the net catch through a smartphone application at close range and distance from the position nets even though we are at home though.

Physiological stress in the smalltooth sawfish: effects of ontogeny, capture method, and habitat quality

ESR Endangered Species Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials ESR 36:121-135 (2018) - DOI: https://doi.org/10.3354/esr00892 Theme Section: Biology and ecology of sawfishes Physiological stress in the smalltooth sawfish: effects of ontogeny, capture method, and habitat quality Bianca K. Prohaska1,*, Dana M. Bethea2, Gregg R. Poulakis3, Rachel M. Scharer3, Ryan Knotek4, John K. Carlson5, R. Dean Grubbs1 1Florida State University Coastal and Marine Laboratory, St. Teresa, Florida 32358, USA 2NOAA National Marine Fisheries Service, Southeast Regional Office, Protected Resources Division, St. Petersburg, Florida 33710, USA 3Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, Charlotte Harbor Field Laboratory, Port Charlotte, Florida 33954, USA 4University of Massachusetts Boston, School for the Environment, Boston, Massachusetts 02125, USA 5NOAA National Marine Fisheries Service, Southeast Fisheries Science Center, Panama City, Florida 32408, USA *Corresponding author: bprohaska@bio.fsu.edu ABSTRACT: Similar to other elasmobranchs, the smalltooth sawfish Pristis pectinata is slow growing, matures late in life, and produces relatively few young, all factors that have contributed to its sensitivity to dramatic population declines from overfishing and habitat loss. Currently, the physiological stress response of these fish to capture or to other physiological challenges such as habitat loss, climatic changes, or pollution is unknown. In the absence of these data, conservation plans may be less effective, making populations susceptible to further declines. We examined basic stress physiology over ontogeny and as a function of capture using different fishing gears. We also examined stress parameters to test whether degraded habitat and water quality from altered habitats may have resulted in chronic stress in juveniles. Results suggested that the stress response to capture by all methods was low, particularly for blood lactate, compared to other elasmobranchs examined to date. Metabolic stress was found to change over ontogeny, with young of the year (YOY) eliciting the highest responses. Glucose, pCO2, bicarbonate, potassium, and hematocrit indicated that gillnet capture induced greater stress responses than longline capture. Significantly higher metabolic stress was observed in YOY and juveniles captured in the 2 nurseries most influenced by anthropogenic activities, the Peace and Caloosahatchee rivers, than in the 2 relatively pristine nurseries in Everglades National Park. Overall, the low physiological stress responses to all capture methods investigated in this study suggest that this species is resilient, which should promote optimism for recovery of the population. KEY WORDS: Pristis pectinata · Stress physiology · Metabolic stress · Chronic stress · Anthropogenic effects · Nursery · Habitat loss · Blood chemistry Full text in pdf format PreviousNextCite this article as: Prohaska BK, Bethea DM, Poulakis GR, Scharer RM, Knotek R, Carlson JK, Grubbs RD (2018) Physiological stress in the smalltooth sawfish: effects of ontogeny, capture method, and habitat quality. Endang Species Res 36:121-135. https://doi.org/10.3354/esr00892 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in ESR Vol. 36. Online publication date: June 29, 2018 Print ISSN: 1863-5407; Online ISSN: 1613-4796 Copyright © 2018 Inter-Research.

Stress-related physiological changes and post-release survival of elephant fish (Callorhinchus milii) after longlining, gillnetting, angling and handling in a controlled setting

Stress-related physiological changes and post-release survival of elephant fish (Callorhinchus milii) in response to fishing capture and handling were assessed by experimental simulation using several different fishing gears for various periods under controlled laboratory conditions and by repeated blood sampling during a subsequent 72-h period to monitor the animals’ condition. Capture and handling caused physiological changes in elephant fish indicated by elevated plasma lactate, potassium and osmolality and immediately depressed plasma glucose. Maximum values of plasma potassium and osmolality occurred mainly immediately after capture (0 h), and maximum levels of plasma lactate at 0 and 3 h. Elevated plasma lactate concentrations returned to baseline values after 12 h of capture. Minimum levels of plasma glucose were recorded 3 h after capture. Hypoglycaemia followed by hyperglycaemia after stress was recorded. Handling alone significantly increased plasma lactate levels. Gillnet capture caused the greatest physiological stress, followed by longline, and then angling. Immediate mortality was 0% for all capture treatments, but delayed mortality varied widely among the capture treatments. Post-release survival rates were 70% (i.e. 30% delayed mortality) for longline and gillnet depending on fishing duration and 100% (i.e. 0% delayed mortality) for hook and line (angling) provided the animals were returned to the water immediately following capture. The best indicators of poor condition in this species were low plasma glucose concentration, red eyes (blood-shot), pale skin and impaired swimming.

Dead tired: evaluating the physiological status and survival of neonatal reef sharks under stress.

Marine protected areas (MPAs) can protect shark populations from targeted fisheries, but resident shark populations may remain exposed to stressors like capture as bycatch and environmental change. Populations of young sharks that rely on shallow coastal habitats, e.g. as nursery areas, may be at risk of experiencing these stressors. The purpose of this study was to characterize various components of the physiological stress response of neonatal reef sharks following exposure to an exhaustive challenge under relevant environmental conditions. To accomplish this, we monitored markers of the secondary stress response and measured oxygen uptake rates ( ) to compare to laboratory-derived baseline values in neonatal blacktip reef (Carcharhinus melanopterus) and sicklefin lemon sharks (Negaprion acutidens). Measurements occurred over three hours following exposure to an exhaustive challenge (gill-net capture with air exposure). Blood lactate concentrations and pH deviated from baseline values at the 3-h sample, indicating that both species were still stressed 3 h after capture. Evidence of a temperature effect on physiological status of either species was equivocal over 28-31°C. However, aspects of the physiological response were species-specific; N. acutidens exhibited a larger difference in blood pH relative to baseline values than C. melanopterus, possibly owing to higher minimum . Neither species experienced immediate mortality during the exhaustive challenge; although, single instances of delayed mortality were documented for each species. Energetic costs and recovery times could be extrapolated for C. melanopterus via respirometry; sharks were estimated to expend 9.9 kJ kg-1 (15% of energy expended on daily swimming) for a single challenge and could require 8.4 h to recover. These data suggest that neonatal C. melanopterus and N. acutidens are resilient to brief gill-net capture durations, but this was under a narrow temperature range. Defining species' vulnerability to stressors is important for understanding the efficacy of shark conservation tools, including MPAs.

Potential for use of accelerometers to monitor green sturgeonAcipenser medirostris(Ayres, 1854) behavior after handling

A pilot study was conducted to test the use of accelerometer tags (coded acoustic transmitters equipped with inertial sensors) to detect changes in green sturgeon activity following gillnet capture and release. Green sturgeon Acipenser medirostris (Ayres, 1854) is listed as threatened under the Endangered Species Act, but is captured as bycatch in both estuarine and coastal gillnet and trawl fisheries. Measured were tailbeat activity and swimming depth of sturgeon (145-167 cm fork length) caught with gillnets in Willapa Bay, Washington during late July 2011. These data were transmitted acoustically over a period of up to 55 d to an array of 16 receivers positioned in the bay. Transmitters were either surgically implanted (n = 2) or attached externally to the dorsal scutes (n = 2). In spite of the small number of fish tagged, over 4,800 data transmissions were obtained, with three fish detected over more than 46 d and in estuaries up to 55 km from the release site. Breakpoint regression analysis indicated that the accelerometers could be used to document discrete changes in activity of the fish after handling. Use of this technology could therefore allow the identification of fishing methods that are most harmful to protected species.

Evaluating the condition and discard mortality of winter skate, Leucoraja ocellata, following capture and handling in the Atlantic monkfish (Lophius americanus) sink gillnet fishery

Abstract Discard mortality (DM) estimates are vitally important to fisheries management. Commercial fishery DM estimates, for example, can help calculate total fishing mortality and biologically acceptable catch limits. The winter skate, Leucoraja ocellata, is the only species within the US portion of the western North Atlantic that is targeted in the skate wing fishery. However, due primarily to fishery regulations, this species is also routinely discarded at sea when daily quotas are reached. While already examined in mobile fishing gears (i.e. bottom otter trawls), DM has not been addressed for the western North Atlantic skate species captured in the sink gillnet fishery, and a conservative 50% DM is used across the species of the skate complex in the management processes. Given this data gap, we estimate the DM of winter skate in sink gillnets under standard commercial fishing conditions. The study methods employed address the issue of cryptic mortality (drop out of dead skates from the gear and depredation prior to retrieval), which can bias DM estimates low for bycatch in passive gear. The degree of injury sustained during capture and handling was found to be a statistically significant predictor of mortality. Overall mean DM rates reached an asymptote within 170 h of captivity, with estimates of 11% and 17% for females and males, respectively. Based on these results, the species appears more resilient to sink gillnet capture and release than previously thought. However, cryptic fishing mortality appeared to be high for extended soak times and likely represents an important focus of future research and management actions.

Behavioral Release Condition Score of Bull and Bonnethead Sharks as a Coarse Indicator of Stress

ABSTRACT Hyatt, M.W.; Anderson, P.A., and O'Donnell, P.M., 2016. Behavioral release condition score of bull and bonnethead sharks as a coarse indicator of stress. Capture and handling stress can induce acidosis and sometimes mortality in sharks. To approximate physiological condition after capture, fisheries researchers may use a behavioral health assessment at release. The goal of this study was to assess the efficacy of the behavioral release condition score (BRCS) in estimating the physiological stress response. The score was tested against changes in acid-base, blood gas, and metabolite analytes (pH, partial pressure of CO2, and lactate) and factors known to influence those analytes (species, capture and handling time, and water temperature) among wild-caught bull (Carcharhinus leucas) and bonnethead (Sphyrna tiburo) sharks. After gill net capture, sharks were processed for tagging, morphometrics, and blood sampling. Blood was sampled immediately before release. At release, a BRCS was assigned as good...

The adenylate energy charge as a new and useful indicator of capture stress in chondrichthyans.

Quantifying the physiological stress response of chondrichthyans to capture has assisted the development of fishing practices conducive to their survival. However, currently used indicators of stress show significant interspecific and intraspecific variation in species' physiological responses and tolerances to capture. To improve our understanding of chondrichthyan stress physiology and potentially reduce variation when quantifying the stress response, we investigated the use of the adenylate energy charge (AEC); a measure of available metabolic energy. To determine tissues sensitive to metabolic stress, we extracted samples of the brain, heart, liver, white muscle and blood from gummy sharks (Mustelus antarcticus) immediately following gillnet capture and after 3 h recovery under laboratory conditions. Capture caused significant declines in liver, white muscle and blood AEC, whereas no decline was detected in the heart and brain AEC. Following 3 h of recovery from capture, the AEC of the liver and blood returned to "unstressed" levels (control values) whereas white muscle AEC was not significantly different to that immediately after capture. Our results show that the liver is most sensitive to metabolic stress and white muscle offers a practical method to sample animals non-lethally for determination of the AEC. The AEC is a highly informative indicator of stress and unlike current indicators, it can directly measure the change in available energy and thus the metabolic stress experienced by a given tissue. Cellular metabolism is highly conserved across organisms and, therefore, we think the AEC can also provide a standardised form of measuring capture stress in many chondrichthyan species.

Respiratory mode and gear type are important determinants of elasmobranch immediate and post‐release mortality

AbstractEstimated declines in shark and ray populations worldwide have raised major, widespread concern about the impacts of global fisheries on elasmobranchs. The mechanisms causing elasmobranch mortality during fisheries’ capture are not fully understood, but we must gain greater clarity on this topic for fisheries managers to develop effective conservation plans to mitigate further population declines. To evaluate how two important factors, respiratory mode and fishing gear type, impact elasmobranch survival, we compiled publicly available data sources on the immediate mortality percentages of 83 species and post‐release mortality percentages of 40 species. Using Bayesian models, we found that sharks and rays captured in longlines had significantly lower immediate mortality than those caught in trawls or gillnets. Our models also predicted the mean total discard mortality (combined immediate and post‐release mortality) percentages of obligate ram‐ventilating elasmobranchs caught in longline, gillnet and trawl gear types to be 49.8, 79.0 and 84.2%, respectively. In contrast, total discard mortality percentages of stationary‐respiring species were significantly lower (longline capture mean = 7.2%, gillnet capture mean = 25.3%, trawl capture mean = 41.9%). Our global meta‐analysis provides the first quantified demonstration of how mortality is affected by these two factors across a broad range of species. Our results and approach can be applied to data‐deficient elasmobranchs and fisheries to identify species that are likely to experience high rates of mortality due to respiratory mode and/or fishing methods used, so that appropriate mitigation measures can be prioritized and investigated.

Basin characteristics and temperature improve abundance estimates from standard index netting of rainbow trout (Oncorhynchus mykiss) in small lakes

Fisheries management often requires efficient methods for assessing populations across a variety of sampling units. We develop a model to assess the status of rainbow trout (Oncorhynchus mykiss) in lakes using a fixed length of multimesh gillnets. We use mark-recapture data to examine how gillnet capture efficiency varies with lake-basin characteristics. Once we accounted for variation in lake-size (net-effort density), we found that capture efficiency was best explained by surface water temperature at the time of gillnetting and lake-basin characteristics (proportion of littoral area). We use Bayesian techniques to fit our model to the data and explore how uncertainty in the estimated parameters leads to uncertainty in population estimates. Our analysis suggests that lake size, temperature and the proportion of littoral area can be used to predict population size using this index netting method. In addition to providing density estimates, our results help to better understand how fish ecology and behavior influences gillnet catches.

Immediate and delayed effects of gill-net capture on acid–base balance and intramuscular lactate concentration of gummy sharks, Mustelus antarcticus

Many sharks are captured as untargeted by-catch during commercial fishing operations and are subsequently discarded. A reliable assessment of the proportion of discarded sharks that die post-release as a result of excessive physiological stress is important for fisheries management and conservation purposes, but a reliable physiological predictor of post-release mortality has not been identified. To investigate effects of gill-net capture on the acid–base balance of sharks, we exposed gummy sharks, Mustelus antarcticus, to 60min of gill-net capture in a controlled setting, and obtained multiple blood and muscle tissue samples during a 72-h recovery period following the capture event. Overall mortality of gummy sharks was low (9%). Blood pH was significantly depressed immediately after the capture event due to a combination of respiratory and metabolic acidosis. Maximum concentrations of plasma lactate (9.9±1.5mmolL−1) were measured 3h after the capture event. Maximum intramuscular lactate concentrations (37.0±4.6μmolg−1) were measured immediately after the capture event, and intramuscular lactate concentrations were substantially higher than plasma lactate concentrations at all times. Sharks in poor condition had low blood pH and high intramuscular lactate concentration, but blood pH does not appear to be a reliable predictor of survival. Suitability of intramuscular lactate concentration as predictor of delayed mortality deserves further investigation.

Distribution of leukocytes as indicators of stress in the Australian swellshark, Cephaloscyllium laticeps

Differential leukocyte counts were taken of blood smears collected from laboratory stressed adult Australian swellsharks Cephaloscyllium laticeps. We calculated the granulocyte (combined count of heterophils and neutrophils) to lymphocyte (G/L) ratio to use as a new physiological indicator of stress for sharks. Animals were captured and stressed using commercial fishing gear (monofilament gillnet and mid-water longline) in a laboratory setting, with blood samples collected prior to capture and at predetermined intervals during a subsequent 72 h recovery period. There was a significant increase in the G/L ratio of 291.14 ± 54.13% at 72 h post-capture during recovery from the 6-h gillnet capture plus 15-mintue air exposure. Six hours of longline capture plus 15 min of air exposure also evoked a significant increase in the G/L ratio of 490.32 ± 294.25% (24 h post-capture) and 590.53 ± 277.65% (72 h post-capture). There was no significant change in the G/L ratio for control sharks that did not undergo capture stress but that experienced an identical blood-sampling regime as captured animals. Our study presents findings of stress-induced changes in leukocyte distribution within the peripheral blood of a shark species brought on by lymphopenia (decrease in lymphocytes) and granulocytosis (increase in granulocytes), and confirms this as a useful measure of the relatively rapid onset of stress in these animals.