Summary
In lakes and rivers of eastern Ontario (Canada) commercial fishers use hoop nets to target a variety of fishes, but incidentally capture non-target (i.e., bycatch) gamefish species such as northern pike (Esox lucius). Little is known about the consequences of bycatch in inland commercial fisheries, making it difficult to identify regulatory options. Regulations that limit fishing during warmer periods and that require frequent net tending have been proposed as possible strategies to reduce bycatch mortality. Using northern pike as a model, we conducted experiments during two thermal periods (mid-April: 14.45 ± 0.32 °C, and late May: 17.17 ± 0.08 °C) where fish were retained in nets for 2 d and 6 d. A ‘0 d’ control group consisted of northern pike that were angled, immediately sampled and released. We evaluated injury, physiological status and mortality after the prescribed net retention period and for the surviving fish used radio telemetry with manual tracking to monitor delayed post-release mortality. Our experiments revealed that injury levels, in-net mortality, and post-release mortality tended to increase with net set duration and at higher temperatures. Pike exhibited signs of chronic stress and starvation following retention, particularly at higher temperatures. Total mortality rates were negligible for the 2 d holding period at 14 °C, 14% for 6 d holding at 14 °C, 21% for 2 d holding at 17 °C, and 58% for 6 d holding at 17 °C. No mortality was observed in control fish. Collectively, these data reveal that frequent net tending, particularly at warmer temperatures, may be useful for conserving gamefish populations captured as bycatch in inland hoop net fisheries.
Methodology
Study site and fish capture
We conducted the study in Lake Opinicon in eastern Ontario, Canada (44°31′N, 76°20′W; mean depth = 4.5 m, surface area = 787 ha; Arlinghaus et al., 2009) in the vicinity of the Queen's University Biological Station during two thermally distinct segments of the spring fishing season: May 23–June 29 (in 2009) and April 15–May 12 (in 2010). Water temperature data were obtained from a temperature probe installed in the lake at a depth of 3.3 m from which readings were automatically recorded each day. We captured the 76 northern pike used for this study by angling from a boat using both trolling and active casting. We angled fish using a variety of artificial lures with one or more barbless treble hooks and we landed the fish as quickly as possible (≤60 s) to minimize capture stress. We landed all fish using smooth rubber landing nets to minimize dermal injury and we quickly de-hooked fish using pliers. Once landed, northern pike were held in large water-filled coolers (1 m × 0.40 m × 0.40 m) that were frequently replenished with lake water (every ∼10 min). The reason for angling fish was to eliminate the variance associated with using the hoopnets to catch fish and the range of times fish would actually enter the net. All fish were angled in this study and angling constitutes an additional acute stressor that would not be present in a hoop net fishery. However, previous studies using northern pike in the same lake and across a similar range of water temperatures revealed that northern pike could be angled with minimal injury (Arlinghaus et al., 2008), physiological recovery was complete within 24 h (Arlinghaus et al., 2009), and that immediate and delayed mortality were negligible (Arlinghaus et al., 2008, Arlinghaus et al., 2009).
Hoop net treatment
We used two hoop net treatments whereby fish were captured by angling and placed in hoop nets that were set for either 2 d or 6 d. We used 10 identical double-ended hoop nets set within 20 m of shore and submerged at 2–3 m depths (see Larocque et al., 2012). All nets were set adjacent to one-another in the same part of the lake. Each hoop net comprised seven 0.91 m diameter steel or wooden hoops positioned 0.5 m apart with two throats per net at the second and fourth hoops. Nets were set by adjoining two hoop nets by their 10.97 m long and 0.91 m high leads with the wings (4.57 m long, 0.91 m high) sewn across the entrance of each net to prevent fish or other taxa from entering (and study animals from escaping). The hoop nets were made from 2.54 cm diamond nylon stretch mesh. Angled pike were placed in the hoop nets through the anchored end (i.e., directly into the last chamber) at a density of up to 4 fish per net. Once northern pike were inserted into nets, the nets were tied shut, re-set and left untouched for either 2 or 6 d. Fish were thus captured by angling for insertion into hoop nets on two dates in each year: April 15 and 19 in 2010 (water temperature = 15.82 °C and 13.91 °C, respectively), and May 23 and 27 in 2009 (16.81 °C and 17.46 °C). The nets were visually inspected daily by snorkelling to monitor mortality rates. Five nets were used each for the 2 and 6 d treatments, both in the warmer (2009) and cooler (2010) treatments.