To mitigate the problem of capturing excessive amounts of juveniles and non-target species in commercial fisheries there have recently been extensive research efforts to improve the size- and species-selectivity of fishing gears, in particular trawl gears (reviewed by Kennelly, 1995; Wileman et al., 1996; Broadhurst, 2000; van Marlen, 2000; Walsh et al., 2002; Valdemarsen and Suuronen, 2003; Graham and Ferro, 2004). Selective fishing has a large potential to reduce fishing pressure on non-target species and juveniles and to reduce discards. Selective fishing gears, however, can be justified only if significant numbers of escaping fish (or other organisms) survive. If most of the fish escaping from trawl codends die, conservation measures specifying minimum mesh sizes or other selective devices are of little value. In the worst case, the effect of this type of unaccounted mortality on fish stocks may be negative because the overall mortality caused by exploitation is underestimated. Hence, quantification of the survival rates of escaping fish is of fundamental importance when selectivity is improved.
The results of studies on post-selection mortality, here called escape mortality, suggest that the mortality associated with capture and escape may be relatively low for many species, particularly for gadoids and flatfishes. However, it is also obvious that not all fish survive the process of capture and escape. In many cases, escape occurs after the fish have been subjected to a wide variety of capture stressors and possible damage through contact with other fish, debris or the gear itself (reviewed by Chopin and Arimoto, 1995; ICES, 2000). Fish that do not die immediately may have their growth and reproductive capacity impaired and may suffer behavioural impairments (e.g. Davis, 2002; Ryer, 2002; 2004). The specific reasons why some fish ultimately die are still poorly understood. There is a substantial need to improve the understanding of this unaccounted mortality, identify its most likely sources and assess its magnitude and impact on the stocks and management of relevant fisheries. Improving the survival of escapees by using better gear modifications and operational solutions requires detailed knowledge of the basic factors affecting stress, injury and mortality of escaping fish.
Measuring the survival of fish escaping from a fishing gear under various fishing conditions is not an easy task. It is subject to high variability and methodological flaws. It is therefore not surprising that the accuracy of the escape mortalities estimated in various studies has been criticized. Until very recently, experimental methodology has been in its infancy, and historically there has been very little standardization of techniques.
It is worth noting that the fate of escaping fish is becoming increasingly important because of a recent strong tendency among fisheries management authorities to increase minimum mesh sizes and/or to use various other controls that improve selection (e.g. Halliday and Pinhorn, 2002). If mortality is high, the benefits of changing selectivity may be largely overestimated. For many important fish species there are insufficient estimates of escape survival to conduct an assessment of its impacts on stocks and fisheries. Failure to quantify the biological impacts of this largely unknown mortality could result in biases in fisheries management decision-making processes.
This study reviews the literature describing the mortality associated with commercial fishing processes and assesses the techniques used to investigate post-selection mortality. Because major bycatch problems are associated with towed fishing gears such as bottom trawls, the focus of the study is confined to these gears. Other commercial fishing gears will be dealt with only to the extent that they are relevant to the main aim of this work. In the following section, potential sources of error in the assessment of mortality are examined and identified, and appropriate methodological approaches are discussed and suggested. The paper then examines how mortality can be decreased through gear modifications and operational changes. Finally, problems in estimating the magnitude and impacts of unaccounted mortality are highlighted, and the approaches and methods by which these mortality estimates may be included in assessment and management processes are demonstrated.
This study focuses on the mortality of fish that actively escape from commercial fishing gears (termed escape mortality), prior to the catch being landed on deck. There is no doubt that this represents a component of total unaccounted fish mortality, but there are very few quantitative data available. Discard mortality (i.e. the mortality of fish actively released or discarded by fishers after capture) is a major component of overall fish mortality (FAO, 1994; 2004; Alverson, 1998). In this study, discard mortality will be addressed only to the extent useful for a general understanding of the capture-induced factors causing stress, injury and mortality of fish. Other types of unaccounted mortality, such as drop-out mortality and ghost fish mortality (see ICES, 1995; 2000; Chopin et al., 1997), and the wider ecological implications of fishing (see Jennings and Kaiser, 1998; Lindeboom and de Groot, 1998; Hall, 1999; Kaiser and de Groot, 2000) are discussed only briefly. Escape and discard mortality in recreational fisheries is beyond the scope of this study, as is the mortality of by-caught marine mammals, reptiles and sea birds. A list of species (and their Latin names) mentioned in this study is presented in Annex 1.