Short communicationSurvival of juvenile herring (Clupea harengas membras) after passing through a selection grid in a pontoon trap
Highlights
► The survival of grid-selected herring in a pontoon trap was examined. ► Groups of grid-selected herring and control herring were confined for seven days. ► The passage through the grid did not affect the mortality of herring. ► Mortality of escaping herring was influenced by water temperature.
Introduction
Several studies have been conducted to examine the survival rates of fish released from fishing gear. (e.g. Chopin and Arimoto, 1995, Suuronen et al., 1996a, Suuronen et al., 1996b, Suuronen and Erickson, 2010). Most of these studies have been on active gear such as trawls and the mortality among escapees has in general been variable and highly species dependent (Suuronen, 2005). One of the species showing the highest mortalities is the herring (Clupea harengus membras). In a study by Suuronen et al. (1996b) as many as 70–100% of herring less than 12 cm in length and 44–83% of herring in the range 12–17 cm selected from the codend of a pelagic trawl died. Suuronen et al., 1996a, Suuronen et al., 1996b argued that the high mortality of herring escaping from trawls is largely due to the exhaustion and physical damage experienced inside the trawl. The passage through a selection device was not the primary cause of injury and death.
Apart from trawling, a common fishing method for herring in the Gulf of Bothnia is the use of traps (Parmanne, 1989, Tschernij et al., 1993). A recently developed trap for this type of fishery is the pontoon trap (Hemmingsson et al., 2008), also called the push-up trap (Suuronen et al., 2006). Since the introduction of the pontoon trap, damages to gear and catch losses caused by grey seals (Halichoerus grypus) have significantly decreased (Lunneryd et al., 2003, Suuronen et al., 2006, Lehtonen and Suuronen, 2010).
However, a problem with herring traps, including the pontoon trap, is that they catch herring of all sizes indiscriminately (Tschernij et al., 1993). By equipping the pontoon trap with rigid grids, an effective exclusion of small herring from the catch can be achieved (Lundin et al., 2011a). But in order to ensure a sustainable fishery, the question must still be resolved as to whether allowing the escape of undersized herring in this way is enough. Are they then in a fit state to survive and grow to maturity?
There are few studies on the survival of fish escaping or released from traps. Siira et al. (2006) showed an average of only 7% mortality among adult Atlantic salmon released from large floating salmon traps along the northern Baltic coast. Fish are not forced to swim into or out of the traps, as both capture and any possible escape requires active behaviour. Therefore, it is unlikely that these fish sustain as much damage and stress during the capture and escape processes as fish that are forced to struggle in order to escape (e.g. from a trawl).A general problem in survival studies of fish escaping from fishing gear is how to measure their mortality rates after escapement and how to collect them without causing any further injury. In the present study, these problems were solved by allowing herring to pass through a grid into a fish chamber where they were kept captive for seven days.
In a stationary trap, caught fish are exposed to environmental stressors before they may escape through a selection device, and it may take them some time to find their way out. A key environmental stressor is temperature, which directly influences the metabolic rates of the organism (Hirst and Bunker, 2003, Folkvord, 2005) and plays a critically important role in the mortality rates of escapees (Suuronen, 2005, Gale et al., 2011).
The aims of the present study were to assess the survival of grid-selected herring in a pontoon trap and to evaluate the effect of ambient temperature variations on mortality rate.
Section snippets
Trials and gear
The study was conducted in the Swedish inshore waters of the Bothnian Sea (61°57′N, 17°22′E). Six trials were performed between the 7th July and the 6th September 2010 with experimental trials using a selection grid alternated with control trials (Table 1).
All herring in the experiment were caught in a herring trap of the same type as used by Lundin et al. (2011a). A single-walled pontoon fish chamber was attached to the trap and used as a fish-holding cage (Fig. 1). The material for the
Results
The mortality of grid-selected herring varied between 2.9% and 12.6%, and the average mortality was 7.0% (Table 1). In the control groups the mortality varied between 7.0% and 45.1%, and the average mortality was 21.2%. Mortality was high in particular in the control sample (no. 2) where there was an abrupt drop in water temperature (Table 1; Fig. 3). Linear regression shows that herring in the control groups had a significantly higher mortality rate (p < 0.001) (Table 2, Table 3).
The ambient
Discussion
This study strongly indicates that a passage through a rigid grid does not in itself affect the short term mortality of herring. It is therefore deemed highly probable that herring would survive an escape from a pontoon trap under commercial fishing conditions.
The most important factor for the short term mortality in the holding period was the ambient water temperature. By far the highest mortality rate was reached during the trial with the greatest temperature fluctuations. During this trial
Acknowledgements
We would like to thank Johan Svedin, Christer Lundin, Mikael Goude and Åke Andersson at Harmångers Maskin and Marin AB who assisted in field work. Thanks to Johanna Lundin, also at Harmångers Maskin and Marin AB for taking care of the administrative tasks in the project. Thanks to Lars Hillström and Arne Fjälling for valuable comments. Thanks also to Erik Peterson for statistical advice and to Graham Timmins for editing the language. The project was financed by funds from the Swedish
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