Elsevier

Fisheries Research

Volume 236, April 2021, 105841
Fisheries Research

Right on target: using data from targeted stocks to reconstruct removals of bycatch species, a case study of longnose skate from Northeast Pacific Ocean

https://doi.org/10.1016/j.fishres.2020.105841Get rights and content

Abstract

Fisheries stock assessments rely heavily on historical catch information to understand how a stock responds to exploitation and make meaningful forecasts under alternative management and environmental scenarios. However, for many bycatch species historical removals are virtually unknown, as a large portion of the catch was discarded at sea. For example, historical discard of elasmobranch species such as skates and sharks have been reported as 95 percent of the total catch based on available data. The longnose skate is one the most abundant groundfishes on the continental slope of the U.S. Pacific Coast by biomass, and the most abundant skate species in the Northeast Pacific Ocean. We developed a method to estimate catch of longnose skate on the U.S. West Coast based on the catch of Dover sole, a co-occurring targeted species with which longnose skate is caught. This method allowed us to reconstruct historical longnose skate catches back to the beginning of the bottom trawl fisheries and improve stock assessment for this species. We also examined the impact of using our method versus other common methods of catch reconstruction to inform stock assessment models, and found that the target-based predictive method produced results that more accurately reflected the life history and typical stock dynamics of elasmobranch taxa. Our method is not limited to the specific case of longnose skate and could be easily adapted for other species and areas.

Introduction

Historical catch information is essential for fisheries stock assessment. Without knowing the catch history, it is difficult to understand how a stock responds to exploitation and make meaningful projections under alterative management and environmental scenarios (Hilborn and Walters, 2003; Branch et al., 2011; King, 2013). Throughout history and into current times, commercial fishery catch statistics have primarily consisted of the portion landed in port, originating from landing receipts, filled out by fish dealers or shoreside catch monitors. For highly prized, economically important fish species that are mostly retained, landings statistics can represent the vast majority of the catch and give a fairly accurate depiction of stock exploitation throughout time (Hilborn et al., 2003; Branch et al., 2011). Species that are not highly prized, but caught together with economically important species, are often discarded at sea, since investment in sorting, processing and cold storage may not be rewarded by high enough returns (Alverson et al., 1994; Kelleher, 2004; Pikitch et al., 1988; Punt et al., 2006; Rogers, 1994). For such species, landings statistics often provide a woefully incomplete or misleading picture of stock exploitation, and the history of actual fishery removals for many stocks is virtually unknown (Hammond and Trenkel, 2005; Harrington et al., 2005).

Elasmobranch species such as skates and sharks, have not been highly valued commercially in most areas around the world (King et al., 2017; Gertseva et al., 2019). With the exception of some sharks that were targeted by short but punctuated fisheries (for vitamin-A rich livers and shark fins, for example), elasmobranchs are primarily taken as bycatch in other fisheries (Bargmann, 2009, Gertseva and Taylor 2011, King et al., 2017). Lack of historical information on elasmobranch exploitation makes it challenging to reliably estimate the current status of a stock, describe its past dynamics and ensure long-term sustainable exploitation of these stocks (Compagno, 1990; Manire and Gruber, 1990; Bonfil, 1994; Rose, 1996). At the same time, life histories of these species are characterized by slow growth, late maturation, low fecundity, and thus low intrinsic rate of increase, making them highly susceptible to overfishing and slow to recover from stock depletion (Smith et al., 1998; Dulvy and Reynolds, 2002; King and McFarlane, 2003; Matson and Gertseva, 2020). Progress in estimating historical catches of elasmobranch species is therefore necessary to enable reliable stock assessment and successful management of these vulnerable species.

Elasmobranch stock assessments have used different approaches to deal with the issue of lack of historical discard information, including assuming only landings to represent stock exploitation (ICES, 2019), and using a single discard rate throughout the time series, or time invariant discard rates applied within large time blocks (Gertseva and Schirripa, 2008). These approaches possess important shortcomings; from underestimating removals if relying only on landings to oversimplification, if applying time invariant discard ratios wholesale across large historical periods. Also, there are shark stock assessments, including the assessment of blue shark (Prionace glauca) in the North Pacific Ocean, that use fishing effort data from logbooks along with catch per unit effort estimates, to calculate total removals of the stock (Kai et al., 2014; Kai, 2016). Logbook data however, are often limited to relatively recent years in many regions and fisheries; this is problematic for assessments in which the modeling period spans back to the unfished equilibrium. Even when logbook records are available, reliability of data vary, depending on reporting rate, record details, and other factors (Sampson, 2002), which causes additional challenges.

Over the past several decades, there have been a number of fishery observer programs instituted around the World, in support of stock assessment, fishery management and conservation (King et al., 2015; Gertseva et al., 2019; ICES, 2019). These programs monitor commercial fishing and collect high-resolution data on discarded and retained catch, in order to estimate total fishing mortality. The observer programs’ data have made it possible to explore relationships among catch of different species that co-occur and caught together as well as analyze additional information, such as depth, location and fishing gear, and also explore approaches to estimate historical discard of bycatch species. There have been attempts to estimate discard of elasmobranchs using statistical models developed based on observer data, while accounting for location, depth and duration of fishing (King et al., 2015). Such approaches, however, rely heavily on rich fishery information in order to function, which limits estimation of discard to a relatively recent, data-rich period, since historical catch records are commonly lacking additional details associated with catch.

In this paper, we present the method developed to reconstruct historical removals of longnose skate (Beringraja rhina) in the Northeast Pacific Ocean, which is a common bycatch species in the groundfish demersal trawl fishery on the West Coast of the United States. Our method relies on known catch records of targeted stocks to predict removals of an associated bycatch species with which it co-occurs, using a statistical relationship developed from data collected by the West Coast Groundfish Observer Program (WCGOP). The method enabled reconstruction of the total catch of longnose skate back to the beginning of the well-established groundfish trawl fishery on the U.S. West Coast. Here we describe the method, present the results and explore implications of using the new method, as well as alternative catch assumptions for stock assessment models. This new method is not limited to the specific case of longnose skate or elasmobranch species but can be adapted for other bycatch stocks lacking historical catch data.

Section snippets

Description of the species and fishery

Longnose skate is one of the most abundant groundfishes on the continental slope of the U.S. Pacific Coast by biomass (Tolimieri and Levin, 2006; Bizzarro, 2015) and the most abundant skate species in terms of biomass and abundance (Gertseva et al., 2019). It is broadly distributed in the Northeast Pacific Ocean, from the southeastern Bering Sea to southern Baja California and the Gulf of California (Snytko, 1987; Eschmeyer and Herald, 1983; Mecklenburg et al., 2002) but is most common off the

Results

The distribution of longnose skate catch by year among target categories (further referred to as “targets”) as observed by WCGOP between 2009 and 2017 is shown in Fig. 3. This bycatch includes both retained and discarded portions of longnose skate catch with no assumptions made about survival of the discard. More than 96 percent of longnose skate are caught within only a few targets, which include the Dover-Thornyhead-Sablefish (DTS) complex, Dover sole (DOVR) complex, Petrale sole (PTRL)

Discussion

Reconstructing historical removals is one of the main challenges for stock assessments of bycatch species, such as elasmobranchs, for which catch is mostly discarded at sea (King et al., 2015; Gertseva et al., 2019; Taylor et al., 2019). Improving catch estimates is therefore an important management priority, addressing which could markedly improve assessments of many fishery stocks. Recently instituted fisheries observer programs provide high quality data on discarded and retained catch and

Declaration of Competing Interest

None.

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