Short-term post-release mortality of skates (family Rajidae) discarded in a western North Atlantic commercial otter trawl fishery

Abstract

Due to market and regulatory factors, Rajidae skates are routinely discarded by commercial otter trawlers in the western North Atlantic. Accounting for post-release mortality is therefore essential to total fishing mortality estimates, stock status and management of this group of fishes. However, despite a presumed species-specific range in tolerance, few studies have investigated the short-term post-release mortality among skates indigenous to the western North Atlantic following capture by mobile fishing gears, and never in the Gulf of Maine. This study addresses this shortfall for the prohibited thorny skate, Amblyraja radiate and smooth skate, Malacoraja senta, and the targeted winter skate, Leucoraja ocellata, and little skate, Leucoraja erinacea. Of 1288 skates evaluated, negligible immediate mortality was observed at the time of capture, even in relation to the largest catches and/or most prolonged tows. However, injury frequency was moderate, with highest levels in the smooth (60%) and thorny (52%) skates. Aside from the smooth skate (59%), 72 h mortality rates were low overall (19% across all species when accounting tow durations indicative of the fishery), with the winter skate (8%) exhibiting the lowest levels. Logistic regression modeling revealed tow duration as the most universal predictor of condition and 72 h mortality, while catch biomass, sex, temperature changes, and animal size also held influence in certain species. Although in general the studied species appear more resilient to trawl capture and handling than previously estimated, interspecific differences must be accounted for when managing this group.

Introduction

Post-release (P-R) mortality estimates for discarded fish are highly important toward, among other management goals, calculating total fishing mortality and biologically acceptable catch limits (Alverson, 1999, Davis, 2002). While complex due to various influencing factors (Davis, 2002) and species differences, pinpointing the specific aspects of the capture and handling process most impactful on P-R mortality can also be exploited to help illuminate best practice scenarios to reduce that mortality (e.g., Parker et al., 2003, Cooke and Suski, 2005).

Skates, like other elasmobranchs, display a K-selected life history (i.e., long lived, late sexual maturation, low fecundity), which make them vulnerable to fishing pressures (e.g., Waring, 1984, Hoening and Gruber, 1990, Sulikowski et al., 2003). These life history characteristics, coupled with global increases in skate landings (e.g., Dulvy et al., 2000, Sulikowski et al., 2005) and high discard rates have led to increasing management and conservation concerns and the need for estimates of fishing mortality. However, only a handful of studies to date have investigated discard mortality rates in skates in general (e.g., Benoît, 2006, Enever et al., 2009, Benoît et al., 2010a), all of which focused on demersal mobile fishing gears. Based on the collective results of these studies, skates suffer moderate P-R mortality (i.e., majority = 40–50%), with factors that influence risk of crushing/compaction and resulting physical trauma (e.g., tow duration, species, animal size, and total catch biomass) appearing to exert the most influence.

The Northeast Skate Complex (NESC) comprises seven species, five of which occur in the Gulf of Maine (GOM) (NEFMC, 2003), where otter trawl is the primary gear responsible for the capture and discard of skates (Sosebee, 1998, NEFMC, 2003, NEFMC, 2011). P-R mortality estimates of trawled skates under a wide range of true commercial conditions are vital toward, among other endpoints, estimating exploitation rates and establishing total allowable landings (NEFMC, 2005, NEFMC, 2011). It is also an important management goal to elucidate interspecific differences regarding both the rate and factors that influence P-R mortality in this complex.

Although a few studies have evaluated discard mortality in trawled NESC species within Canadian waters, sample sizes were either small for certain species (Benoît, 2006), or mortality rates were not distinguished by species (Benoît et al., 2010b). Moreover, intraspecific morphological and biological disparities in NESC species between Canadian and USA stocks (Swain et al., 2006) emphasizes the need for P-R mortality investigations specific to the GOM. Thus, the objective of the present study was to investigate the interspecific acute condition and short-term P-R mortality in four NESC skate species under standard commercial otter trawl conditions in the GOM. The little (Leucoraja erinacea) and winter (Leucoraja ocellata) skates are the subject of directed fisheries, whereas the thorny (Amblyraja radiata) and smooth skates (Malacoraja senta) are prohibited from commercial retention due to overfished stock conditions. A secondary goal of this work was to identify the biological and/or physical factors with the most influence on condition and P-R mortality.