ICES WGEF REPORT 2016 | 639 26 Other issues 26.1 Data needed to assess the bycatch of deep water sharks and possible mitigation measures to reduce bycatch 26.1.1 Background In the past, deep-sea sharks were commonly caught in European mixed deep-water fisheries operating along the Northeast Atlantic continental margin, mainly by deep- water trawlers in the ICES Subareas 4, 5, 6, 7 and 12 and by deep-water longliners in ICES Subareas 8, 9 and 10. The most important species in commercial landings were leafscale gulper shark Centrophorus squamosus, Portuguese dogfish Centroscymnus coelolepis and kitefin shark Dalatias licha. However, many other deep-water shark spe- cies were caught as bycatch, depending on the area and depth of exploitation. In general, the survival of deep-water sharks after commercial fishing operations is considered to be very low. Specimens of leafscale gulper shark and Portuguese dogfish caught by commercial fishing operations generally arrive dead onboard, highlighting that the discard survival is extremely low for the two species. The only indication of some survivorship of deep-water sharks in the Northeast Atlan- tic was observed for leafscale gulper shark caught during a scientific tagging pro- gramme. The survey used deep-water longlines which were laid at depths ranging from 900–1100 m (Rodríguez-Cabello and Sánchez, 2014). In this study, the soak time was restricted to 2–3 hours and the lines were hauled back at a slow speed (0.4–0.5 m.s– 1). It is important to note that these fishing practices are different from those used in commercial fisheries. Similarly, Brooks et al. (2015) also reported some survival of deep-water sharks, but these were also taken in scientific field studies. 26.1.2 Life history and population dynamics Life history characteristics of deep-water sharks typically include a slow growth rate, late age-at-maturity and low fecundity, indicating long generation times (Garcia et al., 2008; Graham and Daley, 2011). Their populations are considered highly vulnerable to commercial exploitation and exhibit low recovery rates when depleted (Pratt and Ca- sey, 1990; Simpfendorfer and Kyne, 2009). Due to their low biological productivity and risk of population decline after exploitation, there are worldwide concerns for their conservation (Kyne and Simpfendorfer, 2010). These concerns were the basis for the adoption, in 2005, of an EU TAC for deep-water sharks which was reduced to zero in 2010 and has remained at zero since then. Despite their overall low resilience to commercial exploitation, species present differ- ent life history traits and dynamics, and these differences should be considered in the assessment and the management of the different species. For example, some species are known to segregate by size, sex and/or maturity stage with different habitat re- quirements during their life (Moura et al., 2014). Table 26.1 summarizes some biological and ecological information of the two most commonly caught species, illustrating their likely different responses to commercial exploitation. 640 | ICES WGEF REPORT 2016 Table 26.1. Summary of the main ecological and biological aspects described for the Portuguese dogfish and the leafscale gulper shark. Leafscale gulper Portuguese dogfish shark (GUQ) (CYO) Comments Overall Worldwide Worldwide Distribution NE Atlantic Mainly caught Commonly caught as There is bathymetric overlap distribution between 600 and 1500 deep as 2000 m, with between the two species, but m deep, with occasional records CYO reaches deeper waters. occasional records down to 2800 m [2]. CYO presents widespread down to 3300 m. For the NE and CE gene mixing across the There is apparently Atlantic there is distribution area. no genetic structure apparently no genetic in the NE Atlantic [1]. structure [4]. NE Atlantic Few records of Few records of For both species, the reasons Ontogenic neonates and small neonates and small for the scarcity of young are stages immature specimens immature specimens not clear, but it is likely that distribution (juveniles) [2]. (juveniles) [2]. they concentrate in nurseries Immature females All adult outside the areas already predominate over reproductive stages surveyed. mature females. can be found within GUQ females are less the same dispersed than males. The Mature males are more broadly geographical region latter is supported by the distributed than of the NE Atlantic [2]. observation of high levels of mature females [2]. male-mediated gene flow [1]. Pregnant females To complete the life cycle GUQ occur at specific females undertake large scale areas, such as off migrations to give birth at Madeira Archipelago specific areas. and at the norther CYO is able to complete the part of NE Atlantic [2, whole life cycle within 3]. Pregnant females different areas of the NE are spatially Atlantic. segregated from the rest of the population. ICES WGEF REPORT 2016 | 641 Reproduction Early ripe follicles are After mating, the GUQ is a continuous breeder present in the ovaries female gonad (i.e. vitellogenesis proceeds in of females in late regresses to a parallel with gestation) with a pregnancy stages. developing stage that unique reproductive season Ovaries of early stage lasts, at least, 1 year All GUQ fully developed pregnant females do [5]. follicles are ovulated not present atretic Ovaries of late-stage CYO is not a continuous follicles. pregnant females breeder, after parturition Ovulation is maximal have a predominance females are likely to enter into during the second of atresia. a resting period quarter of the year Two main ovulation CYO has two ovulation [5]. periods (March-April periods in each year. High frequency of and October- Only a fraction of the mature males in a spent November) [5]. females of CYO is condition observed Significant increase of reproductively active in each from September to females in the first reproductive period October uterine stage and of females carrying the largest intra-uterine embryos in March - April and in October- November [5]. During those periods adult females in different maturity stages coexist. Growth The estimated ages of Estimates of Von The growth rates of both CYO specimens caught to Bertalanffy and GUQ are very low. the west of the British parameters were: Isles varied from 21– growth rate (k) = 70 years. 0.007; L∞ = 128.4 cm Absence of small [7]. specimens restricted the fitting of the growth model [6]. [1] Veríssimo et al. (2012); [2] Moura et al. (2014); [3] Severino et al. (2009); [4] Veríssimo et al. (2011); [5] Figueiredo et al. (2008); [6] Clarke et al. (2002); [7] Moura et al. (2011) 26.1.3 Stock assessment In recent years, both fishery-dependent and fishery-independent data on deep-water sharks taken by Northeast Atlantic deep-water fleets have been limited and considered insufficient to monitor species abundance or biomass trends. These deficiencies are ex- acerbated by the global distribution of some species that occur in waters under many jurisdictions, and also by their specific reproductive strategies. As a consequence of all these data deficiencies, attempts to develop adequate quantitative stock assessments for deep-sea sharks have fallen short of expectations. 26.1.4 Fishery-dependent data Historical, EU fishery data available are restricted to deep-sea sharks of commercial importance, and are of limited use due to the lack of taxonomic and geographic preci- sion. European historical landing data on deep-water sharks are problematic, as for many countries (excluding Portugal) landings were often reported in generic catego- 642 | ICES WGEF REPORT 2016 ries, such as “various sharks nei”. Despite the efforts to retrospectively split the histor- ical landing data into species, no objective rule was considered satisfactory (ICES, 2011). In Portuguese mainland waters (ICES Division 9.a), temporal abundance trends for fe- male Portuguese dogfish for the period 1989–2008 were modelled by a state space model with a Bayesian approach (ICES, 2011; Figueiredo et al., 2013). The model was benchmarked in 2010 (ICES, 2011) and the results showed that at the end of the time series population abundance of both recruited juveniles (70 cm < total length < 101 cm) and adults (total length > 101 cm, i.e., with length larger than the length-at-first ma- turity) were stable (Figure 26.1, upper row). It was also observed that catches of these two life history stages decreased (Figure 26.1, lower row). Both results suggested that the fishing impact on the population inhabiting Portuguese mainland waters was low (Figueiredo et al., 2013). Figure 26.1. Portuguese dogfish – female population abundance estimates (upper row) and esti- mated catch (lower row) in numbers for non-recruited juveniles (left), recruited juveniles (middle) and adults (right), for the first scenario, and their respective 95% credible intervals (doted lines). In the last ten years, and due to the EU restriction measures, registered catches of deep- sea sharks in the NE Atlantic have declined greatly. At the present time, although dis- carding is known to occur in existing deep-water fisheries, these have not been fully quantified. Preliminary ICES estimates are considered uncertain (ICES, 2015). 26.1.5 Fishery-independent data Some fishery-independent data are available but these are often temporally irregular. Fishery-independent data are available from Irish, Scottish and Spanish trawl surveys but none have sufficient geographic and bathymetric coverage to ensure the sampling of all demographic components of the populations. With the exception of the Scottish survey, the remaining surveys are represented by a non-continuous and short time- series (ICES, 2011). The fishery-independent data collected from the Marine Scotland Science deep-water survey in Subarea 6 are available since 1998, at depths ranging from 300–2040 m. Abun- dance indices of leafscale gulper shark and Portuguese dogfish were estimated from ICES WGEF REPORT 2016 | 643 2000 onwards, because only since then were the surveys considered standardized (Fig- ure 26.2).