Investigating spatial overlap between northern sand lance (Ammodytes dubius) and Atlantic cod (Gadus morhua) with video trawl technology

Abstract

Forage fish play important roles in marine ecosystems, but data on abundance and distribution is scarce due to insufficient sampling. We used video trawl survey data to document abundance and distribution of northern sand lance in Stellwagen Bank National Marine Sanctuary and to quantify spatial overlap between sand lance and Atlantic cod, an important predator and commercial species. Video data were analyzed from 18 tows (5 traditional closed cod end tows, 13 open cod end tows) conducted in April 2017 within Stellwagen Bank National Marine Sanctuary. Sand lance and cod were manually counted in videos using custom software or counted on deck in the field following closed cod end tows (cod only). We quantified spatial overlap using the global index of collocation. Data from all tows showed geographic separation between sampled sand lance and cod populations. However, data from only northern sites suggested high overlap and that cod feed on sand lance in northern SBNMS. This work highlights a new use of video trawl data to document abundance and distribution of forage fishes, providing a complementary dataset to better monitor and understand fluctuations in forage species. This information, as well as applications like documenting predator-prey overlap, can support ecosystem-based management.