Stable abundance, but changing size structure in grenadier fishes (Macrouridae) over a decade (1998–2008) in which deepwater fisheries became regulated Francis Neat à , Finlay Burns Marine Scotland-Science, Marine Laboratory, P.O. Box 101, 375 Victoria Road, Aberdeen AB11 9DB, UK article info Article history: Received 7 October 2009 Received in revised form 17 December 2009 Accepted 27 December 2009 Available online 7 January 2010 Keywords: Deepwater fisheries Coryphaenoides CPUE Length frequency Time series abstract A ten-year time series (1998–2008) from a trawl survey of the continental slope of the NE Atlantic was analyzed to assess temporal variation in the abundance and length frequency of seven species of deepwater grenadier fish. This period coincided (in 2003) with the regulation of deepwater fisheries in this area. None of the species declined in numbers or biomass over the period, and 2 species significantly increased. This suggests that the declines in abundance of these deepwater species following the onset of fishing in the 1970s may now have stabilized, albeit at much lower levels than the virgin biomass. Although two metrics of body size (mean length and maximum length) did not show any evidence for consistent decrease over time, there were significant changes in the overall length–frequency distributions. The species found in shallower depths (500 m) had a greater number of larger individuals in 2008 whereas those found deeper (1500 m) tended to have a greater number of smaller individuals. This suggests the presence of a lagged indirect effect of fishing on species that live beyond the actual depths that fishing takes place. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction The continental slopes of the world’s oceans separate the shelf seas from the abyssal plains and are characterized by steep gradients in seabed conditions and enhanced productivity (Gage and Tyler, 1993). Although slope environments represent only a small fraction of the area of the oceans, they support large and diverse fish assemblages. Consequently they are the focus of the majority of deepwater trawl fisheries. Trawlers began fishing the slopes of the NE Atlantic in the1970s where they operated as deep as 1500 m, although most effort was at depths around 1000 m where commercially valuable species such as roundnose grena- dier (Coryphaenoides rupestris), orange roughy (Hoplostethus atlanticus), blue ling (Molva dipterygia), black scabbard (Aphanopus carbo) and deepwater sharks (e.g. Centrophorus squamosus) were most abundant (Gordon, 2001; Gordon et al., 2003). The fish assemblage in this area was also well studied in the 1970s just prior to the establishment of the major commercial deepwater trawl fisheries (Gordon, 1986). As research progressed it became apparent that many deepwater fish attained very old ages, had slow growth rates and were very late to mature, the combination of which made them unproductive and highly vulnerable to over-exploitation (Bergstad, 1995; Koslow et al., 2000; Morato et al., 2006). The example often cited is that of orange roughy (Clark, 2001) which is among the most vulnerable of all deepwater fishes because of its slow growth, late age at maturity (Minto and Nolan, 2006), extreme longevity (Andrews et al., 2009) and aggregating behavior. Furthermore, since the by-catch taken by deepwater fisheries is often large (Allain et al., 2003) and suffers total mortality due to the barotrauma of being brought from great depths, it became apparent that the entire fish assemblage of the slope could be highly vulnerable. Such concerns seem to have been borne out, not only by the decline in catches (Lorance and Dupouy, 2001), but also by two recent studies based on fisheries-independent data. The first, an analysis of data from the western North Atlantic suggested dramatic declines (up to 90%) in abundance of several deep- water slope species since the onset of deepwater fishing (Devine et al., 2006). The second, an analysis of the numerical abundance of the deepwater fish from the slope of the Porcupine bank area in the NE Atlantic suggested that by the 1990s numbers had declined to approximately half of what was estimated prior to exploitation (Bailey et al., 2009). Declines in abundance are, however, inevitable population consequences of exploitation (Haedrich and Barnes, 1997; Pauly et al., 1998; Hilborn et al., 2003), and theoretically a biomass reduction of half may be required to attain maximum productivity (Schnute and Richards, 2002). The declines in deepwater species were ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsri Deep-Sea Research I 0967-0637/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2009.12.003 à Corresponding author. Tel.: + 44 1224 295516; fax: + 44 1224 295511. E-mail address: F.Neat@marlab.ac.uk (F. Neat). Deep-Sea Research I 57 (2010) 434–440