Examining derelict pot impacts on harvest in a commercial blue crab Callinectes sapidus fishery

Highlights

• Derelict blue crab pots can reduce harvests in actively fished pots.

• The effect of derelict pots on harvest differs seasonally.

• Reduced harvests due to derelict pots were more evident in female blue crabs.

Abstract

Pot fisheries occur worldwide with a significant proportion of the gear becoming derelict. Derelict pots induce detrimental ecological and economic impacts, and more recently were found to reduce blue crab harvests in the Chesapeake Bay commercial fishery. We simulated the presence of derelict pots near actively fished pots in seasonal field experiments to quantify the effect derelict pots have on blue crab harvest. Derelict pots reduced harvests by 30% during the summer, but not during the fall. Female blue crab capture rates were consistently lower when derelict pots were present; while capture rates of the less abundant males were not negatively affected by derelict pots. Variable responses to derelict pots may be due to seasonal differences in female and male blue crab behavior and movements. The costly effect that derelict pots have on harvest should be investigated in other pot fisheries to recognize the magnitude and mechanisms behind these impacts.

Introduction

Derelict fishing gear (DFG) is a type of marine debris that consists of any fishing gear that is lost, abandoned, or otherwise discarded, such as nets, pots, trawls, and longlines (Macfadyen et al., 2009; Bilkovic et al., 2016). Several factors contribute to the presence of DFG, such as: operational fishing activities; intentional abandonment; gear conflicts; vessel-gear interactions; vandalism and theft; faulty, degraded, or failed equipment; and storms and weather (Macfadyen et al., 2009; FAO, 2010; Bilkovic et al., 2016; Gilman et al., 2016). A steady increase in fishing effort and improvements in the lifespan of synthetic materials have led to an increase in abundance and persistence of DFG in the marine environment (Macfadyen et al., 2009).

Derelict pots are a prevalent form of DFG that occur globally and are responsible for significant ecological and economic impacts (Guillory, 1993; Macfadyen et al., 2009; Arthur et al., 2014; Bilkovic et al., 2016; Scheld et al., 2016; Wilcox et al., 2016). Continued self-baiting contributes to “ghost fishing” by derelict pots that indiscriminately capture target and non-target animals including finfishes, water birds, turtles, mammals, and other invertebrate species (Guillory, 1993; Havens et al., 2011; Arthur et al., 2014). These animals may become injured, drown, or be consumed by other organisms in the pots (Guillory, 1993; Matsuoka et al., 2005). Bycatch mortalities by derelict pots frequently remove individuals that would otherwise contribute to valuable recreational and commercial fisheries (Guillory, 1993; Bilkovic et al., 2014). Derelict pots may also degrade sensitive habitats (e.g., seagrasses, marshes) by smothering plants, abrading or removing blades of grass, and through scouring areas (Uhrin et al., 2005; Uhrin and Schellinger, 2011; Arthur et al., 2014). Additionally, derelict pots can be a navigation hazard for boaters and can cause costly damage to boat propellers and engines if the buoy line wraps around the propeller (Matsuoka et al., 2005). Attraction towards derelict pots and away from actively fished pots can reduce harvests of target species in a pot fishery, whether or not the target species enter derelict pots (Fig. 1; Scheld et al., 2016).

In 2015, the US commercial blue crab Callinectes sapidus fishery landed over 73 thousand metric tons of blue crab valued at US $220 million (NMFS, 2017). The blue crab is a shellfish of significant ecological (Van Engel, 1958; Virnstein, 1977; Hines et al., 1990) and economic importance on the Atlantic seaboard and Gulf Coast of the United States (Kennedy et al., 2007; NMFS, 2017). Commercial and recreational blue crab fisheries primarily utilize pots, typically 0.6 × 0.6 × 0.6 m rigid wire mesh cubes with an upper and lower chamber (Kennedy et al., 2007). Blue crab pots can continue to fish for two or more years after becoming derelict (Matsuoka et al., 2005; Havens et al., 2008). Blue crab fisheries largely operate in inshore or nearshore environments, leading to a high likelihood of vessel-gear interactions that contribute to increased numbers of derelict pots. Impacts of derelict pots, such as reducing stocks of target and non-target species, decreasing fishery profits, and contributing to user group conflicts, have been well documented in US blue crab fisheries (Guillory et al., 2001; Anderson and Alford, 2014; Bilkovic et al., 2016; Scheld et al., 2016).

The Chesapeake Bay is the largest estuary in the US and is responsible for 30–40% of US blue crab commercial harvests valued at over US $85 million in 2015 (NMFS, 2017). Crab pots are the predominant gear used to harvest blue crabs in both the hard and soft crab fisheries. Following the 2008 US Department of Commerce's declaration of a federal fishery failure in the Chesapeake Bay blue crab fishery, the Virginia Marine Debris Location and Removal Program (2008–2012) was developed and implemented to locate and remove DFG. The program collected data on the abundance and distribution of derelict pots in the Chesapeake Bay (Bilkovic et al., 2014). Subsequent data analyses found that 12–20% of all pots licensed throughout the Chesapeake Bay each year become derelict (approximately 145,000 derelict pots are predicted to be present at any given time; Bilkovic et al., 2016). This high prevalence of derelict pots may intensify negative impacts, affecting fishery resources and the well-being of commercial fishers and local communities who rely on the Chesapeake Bay.

Several past studies have identified the direct loss of biomass in the population of target species (e.g., blue crab, Dungeness crab Cancer magister) due to ghost fishing mortalities by derelict pots over time (Breen, 1987; Guillory, 1993; Havens et al., 2008; Giordano et al., 2010; Antonelis et al., 2011; Anderson and Alford, 2014; Bilkovic et al., 2014; Voss et al., 2015). For example, bycatch mortality of blue crabs in derelict pots is estimated at 20–26 crabs per pot per year (Guillory, 1993; Giordano et al., 2010; Bilkovic et al., 2016). However, limited research has focused on the instantaneous effect on harvest resulting from competition between derelict and actively fished pots. Recent analysis evaluating the Virginia Marine Debris Location and Removal Program and a smaller targeted removal effort the following two winters (2012–2013 and 2013–2014) in Virginia found removal of 34,408 derelict pots increased harvest by 30 million pounds over the course of the programs (Scheld et al., 2016). Derelict pots may compete with nearby actively fished pots by attracting blue crabs away from pots that fishers actively harvest and bait, whether for structure, shelter (Everett and Ruiz, 1993), or foraging for food. Reduction in pot efficiency forces fishers to invest more time, money, and resources to harvest blue crabs in the presence of derelict pots, reducing fishery profits (Scheld et al., 2016). The large-scale analysis of Scheld et al. (2016) used established statistical methods to identify treatment effects in fishery harvests and derelict pot removal data, and suggested a novel economic impact caused by derelict pots. However, data on derelict pot removals was not collected for this purpose and further research is needed to experimentally test the effect that derelict pots have on harvest. The objectives of this study were to (1) experimentally evaluate the effect that derelict pots have on blue crab harvest in actively fished pots, which we hypothesize to be negative, and (2) investigate environmental and temporal factors influencing possible interaction between derelict and actively fished pots.