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Fisheries Behavioral Ecology Program - Newport Laboratory

Swimming Behavior of Pacific Cod and Walleye Pollock in a Simulated Trawl: Effect of Current Speed and Fatigue

figure 7, see caption
Figure 7.  Frame from video of three Pacific cod (mean length = 24 cm) swimming in a circular flume (200 cm diameter).

Behavior is an important and complex factor in fish capture by trawls. Fish react to approaching trawls by escaping or herding into the trawl mouth where they continue to swim forward at the towing speed.

When they become fatigued, fish turn and swim back into the trawl, with smaller (weaker) fish turning back sooner than larger fish. When fatigued fish move back into the body of the trawl and before final capture in the codend, they can encounter gradients of current speed that are slower than the towing speed as well as possible escape routes associated with net structure.

Trawls are often engineered to include bycatch reduction devices (BRDs), which facilitate the escape of nontarget species or sublegal-size fish. Escape can involve both passive sorting and active swimming out of the trawl.

Towing speed and fish fatigue can control swimming behavior in the trawl mouth. However, little is known about the effects of current speed and fatigue on swimming further inside the trawl and how they may affect escape or retention. To design more effective trawl BRDs, probable effects of these variables on swimming behavior in the trawl must be determined.

figure 8, see caption
Figure 8.  Diagram of current speed zones in the circular flume. Sea water flowed into the flume at 38, 57, 76 l min-1 and produced mean (one standard deviation) current speeds (cms-1) in zones A, B, and C.

A circular flume (200-cm diameter, 25-cm depth) (Fig. 7) was constructed to test whether swimming behavior of Pacific cod (21-38 cm total length) and walleye pollock (31-37 cm total length) changed in response to current speed and fatigue. Seawater was introduced into the flume parallel to the tank wall and drained through a center standpipe to create continuous circular currents. The flume simulated current speeds and confinement found in posterior sections of trawls, where fish can accumulate and eventually escape through BRDs or move into the codend.

In the circular flume, fish could chose between zones of faster or slower current speed (Fig. 8). Swimming in the flume was tested with current absent and three maximum current conditions (21, 27, and 42 cm s-1 ).

In an experimental trial, either control fish or fish fatigued to standard condition by chasing with nets were introduced into the flume. Here their swimming behavior was sampled over a period of 6 hours by frequency of tail beats, duration in the faster current zone and the slower current zone, and duration of stationary swimming, upstream swimming, and downstream swimming. Trials were performed in light and dark at 9.0°C and in light at 3.0°C to include a range of conditions found during trawl fishing. Groups of three fish were tested together in 128 experimental trials.

Pacific cod and walleye pollock made swimming choices readily in the circular flume. The most obvious effects on swimming behavior were associated with increased current speed, which resulted in increased swimming speed (frequency of tail beats) and increased duration of stationary swimming (Fig. 9). Fatigue had less of an effect on swimming behavior and resulted in decreased tail beat frequency, decreased duration in the faster current zone, and increased duration of stationary swimming, when compared to control fish (Fig. 9).

figure 9, see caption
Figure 9.  The effects of current strength and fatigue on Pacific cod (1+ and 2+ year) and walleye pollock (2+ and 3+ year) swimming behavior in light and dark at 9°C and light at 3°C. Control (white bar) and fatigued (shaded bar) fish swam for 6 hours in gradients of current, indicated by speed cm s-1) in zone A of the circular flume.  Mean values (+ one standard error) were reported for frequency of tail beats (number min-1), duration in the faster current zone A (s min-1), and stationary swimming (s min-1).

This study showed that prediction of swimming behavior in trawls is probably a complex function of current speed and fatigue. Bycatch reduction devices often modify current speeds in a trawl. The ability for fish to actively choose swimming behavior according to current speed and internal state (fatigue) supports previous field observations that BRDs can actively guide fish through a trawl to escape.

Future research with the aim of increasing effectiveness of trawl BRD designs and prediction of fish escape should characterize current speeds throughout the trawl and test the roles of current speed and fatigue in controlling fish swimming behavior in actual fishing operations.

By Michael W. Davis

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