Mortality of Lingcod Towed in a Net as Related to FishLength, Seawater Temperature, and Air Exposure: ALaboratory Bycatch Study

Abstract

The mortality of discarded bycatch is a critical problem in the management of fisheries worldwide. Little is known about the key principles involved in the mortality of discarded bycatch. These principles are best elaborated under controlled conditions in the laboratory where the actions and interactions of stressors found in fishing practices can be investigated independently. The goal of this study was to investigate the principles involved in the mortality of lingcod Ophiodon elongatus by testing hypotheses concerning the factors that may control trawl bycatch mortality. Lingcod were towed in a net and exposed to increased seawater temperature and to air, two stressors that occur during the processes of trawl capture, retrieval through a thermocline, and landing on deck. Mortality occurred after exposure to more than 45 min in air, after exposure to 4 h towing in a net followed by more than 30 min in air, or after 4 h towing followed by exposure to seawater above 16.0°C for 30 min and air for 15 min. In treatments of equal stressor intensity, smaller fish (41–51 cm total length) had higher rates of mortality than larger fish (52–67 cm). The effects of net towing and air—as well as of towing, increased seawater temperature, and air—were additive. Lingcod bycatch mortality may be reduced by decreasing trawling times and exposure to increased seawater and air temperatures during warmer seasons or by restricting fisheries that produce bycatch to seasons of cooler temperatures. The sorting, handling, and release of bycatch on deck after capture may be conducted in a manner that would probably enhance survival if fish are released within 30 min of capture. Because smaller lingcod had higher rates of mortality, further information about the mortality rates of relevant size‐classes of fish is needed to validate the assumptions of management rules for released, undersized bycatch that are designed to enhance recruitment.

Bycatch rates and initial mortality of paddlefish in a commercial gillnet fishery

“…Likewise, it is generally assumed that mortality in gillnets increases with water temperature for many, if not most, species (e.g., Pacific halibut, Davis and Olla, 2001; spotted seatrout, Cynoscion nebulosus, Murphy et al, 1995). Although the inverse relationship between paddlefish length and initial mortality was not significant, smaller fish of other species have also been shown to suffer higher mortality than larger fish (e.g., lingcod Ophiodon elongatus; Davis and Olla, 2002). In contrast to these expected findings, we are unaware of any published information relating to differential mortality caused by monofilament and multifilament nets.…”

The estimated short-term discard mortality of a trawled elasmobranch, the spiny dogfish (Squalus acanthias)

“…The confounding stress from towing and penning likely explains why larger overall bodysize failed to enhance discard survival in trawled dogfish as was seen in hook-and-lined dogfish herein and previously in trawled teleosts (e.g. Davis and Olla, 2002;Sangster et al, 1996;Suuronen et al, 1996). However, estimated catch-weight was found to be a significant predictor of inter-pen mortality; an effect that remained discernable even after normalizing trawling mortality for perceived pen effects by subtracting out the average hook-and-line (control) mortality.…”

A synthesis to understand responses to capture stressors among fish discarded from commercial fisheries and options for mitigating their severity

“…Other variables, such as environmental conditions, species or lifehistory stage, can all influence air exposure tolerance (Cook et al, 2015). Although some species can tolerate prolonged air exposure, most notably demersal fishes (Davis & Olla, 2002;Haukenes & Buck, 2006), a difference of just 10 seconds was enough to influence fecundity measures in Atlantic salmon (Salmo salar, Richard, Dionne, Wang, & Bernatchez, 2013). When air exposure is combined with other forms of acute capture stress (e.g., exhaustion; see Section 2.4), the combined effect can influence an individual's vulnerability to hypoxia.…”