The recovery of Atlantic halibut: a large, long-lived, and exploited marine predator

ICES Journal of Marine Science ICES Journal of Marine Science (2016), 73(4), 1104– 1114. doi:10.1093/icesjms/fsv266 Original Article The recovery of Atlantic halibut: a large, long-lived, and exploited marine predator M. Kurtis Trzcinski* and W. Don Bowen Bedford Institute of Oceanography, Dartmouth, NS, Canada B2Y 4A2 *Corresponding author: tel: + 1 902 426 9781; fax: + 1 902 426 1506; e-mail: Trzcinski, M. K., and Bowen, W. D. The recovery of Atlantic halibut: a large, long-lived, and exploited marine predator. – ICES Journal of Marine Science, 73: 1104 – 1114. Received 30 July 2015; revised 6 December 2015; accepted 15 December 2015; advance access publication 24 January 2016. Atlantic halibut (Hippoglossus hippoglossus) have a long history of exploitation in the Northwest Atlantic and have gone through several periods of high biomass followed by a population crash. An assessment model using data collected on the Scotian Shelf and southern Grand Banks shows that the population peaked in 1984, then decreased sharply to a low in 1993. Several management measures were taken during the decline, including reductions in total allowable catch and a minimum size limit. Concurrently, removals by the otter trawl fishery were drastically reduced following the collapse of the cod (Gadus morhua) fishery. In 2003, recruitment increased and continued to be high for 6 years. Fishing mortality rates were moderate in the late 1990s and 2000s and the population increased. By 2009, the Atlantic halibut population was highly productive with both high biomass and high levels of recruitment. The coincidence in the timing of population recovery and management actions indicates that effective management contributed to the recovery of Atlantic halibut. Keywords: Atlantic halibut, fisheries management, population decline, population recovery. Introduction Overexploitation has led to the extinction or commercial extinction of many marine populations (Harnik et al., 2012). There is considerable debate in the literature about the rate of extinction and the rate of population recovery (e.g. Myers and Worm, 2003; Sibert et al., 2006; Worm et al., 2009; Hutchings et al., 2010; Lotze et al., 2011; Pauly et al., 2013; NOAA, 2014). Over the past 50– 100 years, fisheries science has made great advances (Beverton and Holt, 1957; Ricker, 1958, 1975; Hilborn and Walters, 1992; Quinn and Deriso, 1999), but important gaps in knowledge remain. In particular, fisheries scientists have struggled to determine what levels of exploitation are sustainable for a population and the ecosystem it inhabits (Hilborn, 2002; Shelton and Sinclair, 2008; Link et al., 2011). One of the great tragedies in fisheries was the overexploitation of Atlantic cod (Gadus morhua) in the Northwest Atlantic (Myers et al., 1997), which led to economic disaster in small fishing communities in eastern Canada and to an entirely different functioning of the ecosystem (Frank et al., 2005, 2011). The collapse of the Atlantic cod population is only one example of many (Hutchings et al., 2010). However, a balanced view acknowledges that there are examples of highly effective science-based fisheries management (Cunningham and Bostock, 2005) where many stocks are not overfished (Worm et al., 2009), including the ca. 90 years of management of the Pacific halibut (Hippoglossus stenolepis) fishery by the International Pacific Halibut Commission (Gates, 2005). Population declines can occur because of overfishing or changes in the environment. A manager’s most effective, and often only, tool to halt the decline of an exploited population is to reduce exploitation. There are several ways to reduce exploitation (e.g. effort control and spatial reserves), and reducing the total allowable catch (TAC) is perhaps the most common approach. Some populations have recovered to rebuilding targets following management action aimed at improving population productivity. NOAA (2014) reports that 37 stocks have been rebuilt (B/BMSY . 1) since 2000 as a result of good management, favourable environmental conditions, or both. However, Murawski (2010) reports that only 1% of the world’s depleted stocks have recovered. There are an alarming number of examples where the TAC was reduced, but fish populations were slow to recover (Hutchings, 2000; Hutchings and Baum, 2005; Hutchings et al., 2010; Neubauer # International Council for the Exploration of the Sea 2016. All rights reserved. For Permissions, please email: Effective management contributed to the recovery of Atlantic halibut 1105 et al., 2013). Slow population recovery indicates several gaps in knowledge or poor assumptions: (i) the productivity of the population at low population size was overestimated or changed with environmental conditions, (ii) the theory used to predict recovery is missing essential features of fish life history, (iii) the reduction in the TAC was not sufficiently large enough to promote recovery, (iv) unreported catch is large enough to prevent recovery, and (v) the broader effects of the biological community on ecosystem function and productivity are not adequately understood. Atlantic halibut (Hippoglossus hippoglossus) is a large flatfish found throughout the North Atlantic and is similar in size, habits, and life history to its congener, the Pacific halibut (Trumble et al., 1993). Spawning occurs in deep waters along the continental shelf. Within several months, juveniles undergo the unique metamorphosis of flatfish whereby the right side migrates to become the top of the fish. Halibut grow rapidly, initially consuming a diverse array of invertebrates, then switch to piscivory between the ages of 3 and 5 (Cook and Bundy, 2010). Juvenile halibut grow rapidly (ca. 10 cm year21) until maturity (Armsworthy and Campana, 2010), which is estimated to be 77 cm for males (ca. age 5) and 119 cm for females (ca. age 9; Table 1 in Trzcinski et al., 2011a). Large halibut can be extremely fecund. In Norway, a 90.7-kg (ca. 195 cm) female produced over 2 million eggs (Lønning et al., 1982), and in another study, a 195-cm female produced ca. 7 million eggs (Haug and Gilliksen, 1988). Adults are sexually dimorphic, grow to large size (females up to 2.5 m), are long-lived (.40 years), and are late maturing (ca. age 9 for females; Sigourney et al., 2006; Armsworthy and Campana, 2010). The large-bodied, late-maturing life history strategy of Atlantic halibut places a biological limit on reproduction, so that the potential rate of population recovery could be slow; however, individual growth is fast, ca. 10 cm a year, and females are highly fecund, which potentially make the population resilient and able to quickly rebound from low population size. Tagging data show that Atlantic halibut move long distances and have a broad geographic distribution (Trumble et al., 1993). The population on the Scotian Shelf has some directional movement from the southwest to the northeast (Stobo et al., 1988), and a (...truncated)