Date Published: March 11, 2019
Publisher: Public Library of Science
Author(s): Neil Anders, Mike Breen, Jostein Saltskår, Bjørn Totland, Jan Tore Øvredal, Aud Vold, Vitor Hugo Rodrigues Paiva.
The release of unwanted fish from purse seines whilst still in the water is termed slipping and may lead to significant mortality following release. The objective of this study was to determine the fish welfare implications of a new slipping methodology in which fish are released via a discharge opening formed in the bunt end of the purse seine net. Video analyses of collective and individual level fish behaviour were undertaken in the Norwegian mackerel and herring purse seine fisheries, to quantitively describe slipping behaviour and to determine its driving factors. The majority of fish escaped the purse seine with the schooling structure intact as part of large groups towards the end of slipping process, increasing their speed following escape. However, there was also a tendency (24% of all escapes) to escape in a manner likely to impact negatively upon their welfare, with a breakdown in schooling structure and physical contact with the fishing gear and conspecifics. The tendency to express such welfare compromising behaviour was higher for mackerel than for herring, but was also influenced by the vessel releasing the fish, the amount of fish being slipped, how long the discharge opening had been open and the particular slipping event. These results provide important information for future science-based development of welfare friendly slipping practises.
Purse seining is a widespread , effective  and relatively fuel efficient [3–6] capture method for small pelagic schooling species. However, it is not without challenges. A lack of suitable monitoring technology means that skippers typically lack detailed information regarding school size and characteristics prior to setting the net . This can lead to discarding when resulting catches are undesirable in some way. Discarding can be detrimental to sustainable fishery management as discarded fish may die [8,9], which can introduce uncertainty into stock assessment if not properly accounted for [10–12].
Observations of behaviour were collected from 39 slipping events across 8 different trips (Table 3). Of these events, 4 (10%) contained no usable behavioural data (Fig 1), either because the vessel failed to encircle the target school and the net was empty, or because no behavioural footage was recorded on the cameras. The discharge cameras were deployed on all events, while circumstances onboard meant that the drop camera was deployed on only 27 events (70% of all observed casts). Of the usable footage, the majority of observations (88%, n = 31) represented complete slipping events rather than partial slips. For both vessels combined, mean slipped amount was 158t (range: 1–1200t), while mean (± SD) width, depth and area of the discharge channels was 11 ± 2m, 7 ± 3m and 50 ± 25m2, respectively. Observations from individual slipping events consisted wholly of either mackerel or herring; mixed species catches were not encountered.
Efforts to maximise post slipping survival through better catch control earlier in the capture process (such as those described by  and ), may be negated if the slipping process itself further compromises the welfare of released catches. As such, it is important to understand how slipping methodology affects welfare. The results of this study describe the behaviour of mackerel and herring during slipping from purse seines, and highlight factors affecting the observed behaviour. To our knowledge, these observations are the first to describe fish behaviour whilst slipping from purse seines in the field.