Model of Swordfish Daytime Vertical Habitat May Aid in Daytime Harvest of Swordfish and Avoidance of Sea Turtles

A key factor in reducing bycatch in pelagic longline fisheries is to understand the spatial and temporal distributions of the affected marine fauna, including fish species targeted by the fishing gear and non-target fish and protected species caught incidentally. In Hawaii, a shallow-set longline fishery targets swordfish but also catches other species including protected sea turtles and seabirds. Research that may help reduce incidental capture of turtles in the swordfish fishery was recently reported by Melanie Abecassis of the University of Hawaii Joint Institute for Marine and Atmospheric Research (JIMAR), Heidi Dewar of the NOAA Southwest Fisheries Science Center, Donald Hawn of JIMAR, and Jeff Polovina of PIFSC. The team completed a paper describing their study of the daytime foraging depth of swordfish. The study is based on data from an 8-year tagging program using 28 Wildlife Computer pop-up archival tags deployed on swordfish in the North Pacific and covering latitudes from 1° S to 44° N and longitudes from 111° W to 154° W.

Based on a trio of environmental indices, the GAM prediction model accounted for 77% of the variation in observed mean 
               daytime depth of tagged swordfish.  White dots: predicted values; Black dots: observed values.   Results are shown for three 
               tagged swordfish.
Based on a trio of environmental indices, the GAM prediction model accounted for 77% of the variation in observed mean daytime depth of tagged swordfish. White dots: predicted values; Black dots: observed values. Results are shown for three tagged swordfish.

Of the 28 tags deployed, 5 were recovered, providing a full archival record of the fish's whereabouts. The data showed that when swordfish did not engage in daytime basking behavior near the sea surface, they remained within a narrow range of light level at all hours, suggesting that swordfish stay within a sound scattering layer (SSL) to feed during both day and night. Daytime mean depth of non-basking swordfish ranged from 32 to 760 m. Seventy-seven percent of the variation in daytime mean depth could be explained with a generalized additive model (GAM) that used 3 environmental indices as predictors: satellite-derived surface chlorophyll as a proxy for light at depth, oxygen at 400 m obtained from the World Ocean Atlas, and temperature at 400 m inferred from the tag data.

Used in a predictive mode, the GAM model generated a basin-wide map of swordfish daytime mean depth. The model predicted swordfish occupying a habitat exceeding depths of 600 m to the north of Hawaii, shoaling to depths of 300 m off the coast of California. This information could improve the ability of longliners to locate and catch swordfish during the daytime and potentially allow them to switch from making shallow night sets that result in interactions with sea turtles. This modeling of depths occupied by swordfish in effect defines the habitat of their prey, giving us insight into the vertical behavior of those mid-trophic level organisms inhabiting the SSL. The model could be easily applied to other deep-foraging species.

The GAM model was used to predict a map of daytime mean depth (m) for North Pacific swordfish in January 2010 (top) and July 
               2010 (bottom). Numbered lines represent contours of predicted depth.
The GAM model was used to predict a map of daytime mean depth (m) for North Pacific swordfish in January 2010 (top) and July 2010 (bottom). Numbered lines represent contours of predicted depth.