Hitting Rock Bottom? Modelling Essential Fish Habitat for Commercially Important Hawaiian Bottomfish

Hawaii’s deepwater handline fishery is the second most valuable fishery in Hawaii.  It targets a number of deepwater species including snappers (Lutjanidae), groupers (Serranidae) and jacks (Carangidae).  Many of these species have a relatively high age of maturity, long life span and slow growth rate making them highly susceptible to overfishing.  To ensure long term-sustainability of bottomfish stocks, the State of Hawaii Department of Land and Natural Resources implemented bottomfish restricted fishing areas.   However, defining geographic areas incorporating essential fish habitat has been difficult with little species-specific habitat data available.  To address this knowledge gap we used species distribution modelling to identify key environmental variables structuring deep water bottomfish distributions.  The three most commercially valuable species were examined in detail; Onaga (Etelis coruscans), Ehu (Etelis carbunculus) and Opakapaka (Pristipomoides filamentosus).  Environmental variables included high-resolution hydroacoustic data including bathymetry, backscatter and terrain derivatives (e.g. slope, aspect and rugosity).  Preliminary analyses indicate significant differences in the way in which the three species utilise their environment.  Onaga had the broadest depth range (> 211 metres).  However, within this depth range presence was restricted to hard and high relief substrates explaining 40% of its distribution.  The distribution of Ehu was defined by depth with 64.3% of the variation in its occurrence attributed to depths greater than 246.1 metres.  In contrast, opakapaka, utilised depths less than 246.9 metres and preferred high slope, unconsolidated sediments with a north eastern aspect (explaining 47.8% of its distribution).  This distribution is likely to be related to the distribution of its prey.  Opakapaka is a macroplanctivore predominantly feeding on small pelagic crustaceans.  These prey items are known to be affected by local upwelling along exposed reef edges and slopes.  This modelling provided a more detailed understanding of species distributions and landscape linkages.  Information of this nature will improve the species-specific essential fish habitat definitions, improve marine protected area designation and aid in the sustainable management of Hawaii’s deep bottomfish resource.