More than 50% of the world's total marine catch (approximately 81 million tonnes) is harvested using towed fishing gears (i.e. Danish seines, dredges and otter and beam trawls). As for all methods, the total fishing mortality of these gears comprises the reported (landed) and unreported catch and other unaccounted, collateral deaths due to (i) avoiding, (ii) escaping, (iii) dropping out of the gear during fishing, (iv) discarding from the vessel, (v) ghost fishing of lost gear, (vi) habitat destruction or subsequent (vii) predation and (viii) infection from any of the above. The inherent poor selectivity of many towed gears, combined with their broad spatial deployment, means that there is considerable potential for cumulative effects of (i)-(viii) listed above on total fishing mortality, and subsequent wide-scale negative impacts on stocks of important species. In this paper, we develop a strategy for minimizing this unwanted exploitation by reviewing all the primary literature studies that have estimated collateral, unaccounted fishing mortalities and identifying the key causal factors. We located more than 80 relevant published studies (between 1890 and early 2006) that quantified the mortalities of more than 120 species of escaping (26 papers) or discarded (62 papers) bivalves, cephalopods, crustaceans, echinoderms, elasmobranches, reptiles, teleosts and miscellaneous organisms. Seven of these studies also included the estimates of mortalities caused by dropping out of gears, predation and infection [(iii), (vii) and (viii) listed above]. Owing to several key biological (physiology, size and catch volume and composition), environmental (temperature, hypoxia, sea state and availability of light) and technical (gear design, tow duration and speed) factors, catch-and-escape or catch-and-discarding mechanisms were identified to evoke cumulative negative effects on the health of most organisms. We propose that because the mortalities of discards typically are much greater than escapees, the primary focus of efforts to mitigate unaccounted fishing mortalities should concentrate on the rapid, passive, size and species selection of nontarget organisms from the anterior sections of towed gears during fishing. Once maximum selection has been achieved and demonstrated to cause few mortalities, efforts should be made to modify other operational and/or post-capture handling procedures that address the key causal factors listed above.

Environmental DNA (eDNA) has revolutionized our ability to identify the presence and distributions of terrestrial and aquatic organisms. Recent evidence suggests the concentration of eDNA could also provide a rapid, cost-effective indicator of abundance and/or biomass for fisheries stock assessments. Globally, fisheries resources are under immense pressure, and their sustainable harvest requires accurate information on the sizes of fished stocks. However, in many cases the required information remains elusive because of a reliance on imprecise or costly fishery-dependent and independent data. Here, we review the literature describing relationships between eDNA concentrations and fish abundance and/or biomass, as well as key influencing factors, as a precursor to determining the broader utility of eDNA for monitoring fish populations.We reviewed 63 studies published between 2012 and 2020 and found 90% identified positive relationships between eDNA concentrations and the abundance and/or biomass of focal species. Key influencing biotic factors included the taxon examined as well as their body size, distribution, reproduction, and migration. Key abiotic factors mostly comprised hydrological processes affecting the dispersal and persistence of eDNA, especially water flow and temperature, although eDNA collection methods were also influential. The cumulative influence of these different factors likely explains the substantial variability observed in eDNA concentrations, both within and among studies. Nevertheless, there is considerable evidence to support using eDNA as an ancillary tool for assessing fish population abundance and/or biomass across discrete spatio-temporal scales, following preliminary investigations to determine speciesand context-specific factors influencing the eDNA abundance/biomass relationship.Advantages of eDNA monitoring relative to other approaches include reduced costs, increased efficiencies, and nonlethal sampling.

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Gillnets and traps often are considered to have fewer holistic environmental impacts than active fishing gears. However, in addition to the targeted catches, gillnets and traps still cause unwanted mortalities due to (i) discarding, (ii) ghost fishing of derelict gear, (iii) depredation, (iv) escaping or dropping out of gear, (v) habitat damage, and potentially (vi) avoiding gear and predation and (vii) infection of injuries sustained from most of the above. Population‐level concerns associated with such ‘unaccounted fishing mortalities’ from gillnets and traps have been sufficient to warrant numerous attempts at mitigation. In this article, we reviewed relevant research efforts, locating 130 studies in the primary literature that concomitantly quantified mortalities and their resolution through technical modifications, with the division of effort indicating ongoing concerns. Most studies (85) have focused on discard mortality, followed by ghost‐fishing (24), depredation (10) and escape (8) mortalities. The remaining components have been poorly studied (3). All problematic mortality components are affected by key biological (e.g. species), technical (e.g. fishing mechanisms) and/or environmental (e.g. temperature) factors. We propose that these key factors should be considered as part of a strategy to reduce impacts of these gears by first assessing modifications within and then beyond conventional configurations, followed by changes to operational and handling practices. Justification for this three‐tiered approach is based not only on the potential for cumulative reduction benefits, but also on the likely ease of adoption, legislation and compliance.

Since humans began fishing (at least 90 000 years ago), fishing technology has developed with the objective of trying to catch the greatest quantities of fish possible, of an ever‐increasing variety. Fishing technology has evolved from simple harpoons and hooks to the industrial factory trawlers of the 20th century. After millennia of assuming that seafood resources are inexhaustible, and centuries of somewhat muted concerns that advanced fishing technologies may have detrimental impacts on stocks and ecosystems, the last century has seen advances in fishing technology blamed as a major cause of the current over‐exploitation of fish stocks. It has mainly been during the last few decades that fishing technologists have begun to focus on more conservation‐orientated goals. This occurred initially in response to concerns over the by‐catch of charismatic species (like dolphins in tuna purse‐seines), but quickly broadened to address concerns over the discarding of not‐so‐charismatic species (like juvenile fish killed by shrimp trawling). To ameliorate these issues, technologists and commercial fishers successfully developed various innovative gear‐based and operational solutions. The steps involved in successfully reducing by‐catches have tended to follow a certain incremental framework involving identification of problems using observer programmes, developing technological solution to these problems, experimentally testing these solutions, implementing these solutions throughout industry and finally gaining acceptance of the solutions from concerned interest groups. Most recently public concern has broadened once again from by‐catch issues to encompass a much wider context involving the impacts of fishing on entire ecosystems, i.e. the impacts of fishing on all species affected – not just those species caught, retained or discarded. As a consequence, there have been many calls for ecosystem‐based fisheries management to ensure that fisheries operate under the principles of ecologically sustainable development. Scientists are gradually filling the gaps in our knowledge about how fishing affects whole ecosystems but, because of the scales and complexities involved, such studies are usually difficult, expensive and of long duration. While this descriptive work is difficult, finding solutions to any identified problem is an even greater challenge, particularly for fishing technologists. The easiest solutions to such problems involve rather draconian management strategies like closures. A less extreme alternative involves the development of new technologies that reduce the impacts of fishing on ecosystems – in a similar way as that done to reduce by‐catch problems. Innovations like altering ground‐chains, footropes, sweeps and trawl doors have been suggested as possible ways to ameliorate the environmental damage done by trawling, but such research is still very much in its infancy. Nevertheless, the recent history of fishing technology is chequered with successfully meeting such challenges, giving one confidence...

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Butcher, P. A., Leland, J. C., Broadhurst, M. K., Paterson, B. D., and Mayer, D. G. 2012. Giant mud crab (Scylla serrata): relative efficiencies of common baited traps and impacts to discards. – ICES Journal of Marine Science, 69: . This study was initiated in response to a scarcity of data on the efficiency, selectivity and discard mortality of baited traps to target Scylla serrata. Five replicates of four traps, including “hoop nets”, rigid “wire pots”, and collapsible “round” and “rectangular” pots were deployed for 3, 6 and 24 h in two Australian estuaries. Trapped S. serrata were “discarded” into cages and monitored with controls over 3 d. All S. serrata were assessed for damage, while subsets of immediately caught and monitored individuals had haemolymph constituents quantified as stress indices. All traps retained similar-sized (8.1–19.1 cm carapace width) S. serrata, with catches positively correlated to deployment duration. Round pots were the most efficient for S. serrata and fish—mostly Acanthopagrus australis (3% mortality). Hoop nets were the least efficient and were often damaged. No S. serrata died, but 18% were wounded (biased towards hoop nets), typically involving a missing swimmeret. Physiological responses were mild and mostly affected by biological factors. The results validate discarding unwanted S. serrata for controlling exploitation, but larger mesh sizes or escape vents in pots and restrictions on hoop nets would minimise unnecessary catches, pollution and ghost fishing.