Elsevier

Fisheries Research

Volumes 127–128, September 2012, Pages 83-87
Fisheries Research

Short communication
Survival of juvenile herring (Clupea harengas membras) after passing through a selection grid in a pontoon trap

https://doi.org/10.1016/j.fishres.2012.05.009Get rights and content

Abstract

A common problem in the Baltic Sea and Gulf of Bothnian herring trap fishery is the bycatch of undersized Baltic herring (Clupea harengus membras). By equipping a pontoon trap with a rigid grid with vertical bars, a substantial proportion of the undersized herring can escape from the gear by swimming through the grid. However, it has not been clear whether or not the young fish are injured by their passage through a grid and what their fate is. The purpose of this study was to investigate the short term mortality of herring after passage through a grid. To capture young herring, a selection grid was mounted at the entrance to the fish chamber of a pontoon trap, so that only young herring (approximately 12–18 cm in length) could swim in. The trap was then closed and the herring were confined in the fish chamber for seven days before assessing survival rates. The same procedure was performed with control herring which were trapped without passing through any grid. The whole trial was repeated three times. On average about 7% of the young herring selected through a sorting grid died during the experiment. The results demonstrated that passing through a rigid grid did not affect the short term mortality of young herring during the seven days in captivity. The highest mortality (45%) was in fact observed in one of the control groups presumably in connection with extreme temperature fluctuations.

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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