Summary
To date, most studies of commercial fisheries bycatch have focused on mortality at time of capture as an endpoint. However, sub-lethal indicators of organismal condition have the potential to reveal mechanisms associated with mortality (both at time of capture and post-release) and opportunities for improving fish welfare. In this study, we simulated commercial fishing efforts in inland lakes with fyke nets during a typical fishing season (early April to late June) in southeastern Ontario, Canada, where bycatch of non-target fish species had previously been documented. Using non-target gamefish (i.e., largemouth bass [Micropterus salmoides, Lacépède], northern pike [Esox Lucius, L.]), as well as a target species (i.e., bluegill [Lepomis macrochirus, Rafinesque]), we examined the sub-lethal consequences of capture (e.g., blood physiology, reflex impairment, and injury) and compared the effects of being retained in the net for two different durations (i.e., two or six days) over a range of water temperatures (i.e., 3–28 °C). Sub-lethal physiological disturbances (i.e., blood glucose and lactate) in largemouth bass and bluegill tended to be greater at higher water temperatures. However, fish retained for six days generally did not exhibit greater stress than those retained for two days, with the exception of plasma glucose in largemouth bass. Reflex impairment was similar among temperature and retention periods. Fish retained in nets experienced a range of injuries (including fin frays, scale loss, and mouth damage) that had the potential to facilitate the development of opportunistic pathogenic infections. Greater incidences of injury on fish bycatch tended to be associated with higher temperatures and longer retention. To reduce physiological disturbances and injury that could lead to delayed mortality, we suggest that regulations for inland commercial fishers require them to check their nets more frequently as water temperatures increase. We suggest that future studies of bycatch incorporate sub-lethal endpoints given that they serve as an objective measure of fish welfare and can provide quantitative mechanistic information to support management actions.
Methodology
Study area
Newboro Lake (44°38′ W 76°20′ 0) is a target lake for commercial fishers in southeastern Ontario, Canada, and thus was chosen for our simulated commercial fishing efforts. The lake is shallow (mean depth = 3 m, maximum depth = 24 m, surface area = 787 hectares) and is characterised by submerged stumps and heavy macrophyte cover. The fish community of Newboro Lake is dominated by centrarchids.
Study gear and deployment procedures
Fyke nets were set in Newboro Lake during the spring of 2009 after “ice-off” (April 4) until the end of the legal fishing season (June 20) to allow sampling over a range of water temperatures (see Larocque et al. (2012) for full description of nets including a diagram and deployment methods). Briefly, nets consisted of eight 0.8 m diameter wooden hoops positioned 0.5 m apart. There were three narrowing throats per net, on the first, third and fifth hoop of the net. Each net had two wings (2.9 m long and 0.8 m high) and a lead (11 m long and 0.8 m high) attached to the front hoop. These throats funnel fish into the net where they become trapped and are unable to find their way out. All nets, wings and leads were constructed with 5.08 cm stretch nylon mesh. To emulate the commercial fishery, all nets were set in tandem by adjoining two hoop nets by their leads, with the net openings facing each other and extending the wings to a 45° angle from the entrance of the net. Nets were deployed for either two or six days to examine the sub-lethal effects of fish captured under current regulations, as well as potential options for changes to the regulations, respectively. For nets deployed for two days the nets were open and fishing for two days. For nets deployed for six days, the nets were open for the first two days, after which the nets were sewn shut holding the fish for an additional four days. Therefore, two treatments of duration were used in this study; fish held up to two days and fish held for four to six days.
Catch metrics and sub-lethal evaluations of fish condition
A variety of sub-lethal metrics (blood chemistry, reflexes, injury) were measured on largemouth bass (bycatch), northern pike (bycatch), and bluegill (commercial target species). While bluegill are a target species for the commercial fishery, they were included in sub-lethal analyses as we were confident that they would be captured over a wide range of water temperatures and in numbers that would ensure reasonable sample sizes. We regard them as a reasonable proxy for sub-adult largemouth bass given that they are confamilials and of a similar size. Moreover, although bluegill are legally one of the possible target species, some of the commercial fishers only retain some species or sizes of target fish and therefore some are discarded. Not all measures were obtained from each species based on availability of fish and in some cases because of existing validations of the techniques used in this study.