Impact of light on catch rate of four demersal fish species during the 2009-2010 U.S. west coast groundfish bottom trawl survey

To determine the influence of light on catch of demersal fish, we examined the relationship between near-bottom light levels, catch rates, and catch probability for four abundant groundfish species well represented in annual bottom trawl surveys on the U.S. west coast: arrowtooth flounder (Atheresthes stomias), greenstriped rockfish (Sebastes elongatus), longnose skate (Raja rhina), and Pacific hake (Merluccius productus). Relative downward irradiance was measured with MK-9 archival tags during annual trawl surveys along the U.S. west coast in 2009 and 2010. Near-bottom light levels were recorded for 818 hauls at depths less than 400 m. Significant linear relationships were observed between catch per unit effort (CPUE, kg ha−1) and near-bottom light (P < 0.05). CPUE of arrowtooth flounder, longnose skate, and Pacific hake was negatively related to near-bottom light. For these species, CPUE decreased 16-22% per unit increase in log10 light (μE m−2 s−1). CPUE of greenstriped rockfish increased 39% per unit increase in log10 light. Light, depth, and latitude explained 15-47% of the variance in CPUE for the four species. Catch probability was significantly related to light, depth, latitude, and relative time of day (P < 0.05). For all species, catch probability varied inversely with light when depth was less than 200 m. At depths of 200-300 m, catch probability increased with light for arrowtooth flounder and greenstriped rockfish. Catch probability for Pacific hake decreased slightly at depths greater than 200 m while longnose skate was relatively unaffected by light at these depths. We used these relationships to explain the variability in catch rates for individual species within bottom trawl surveys. By influencing the density and distribution of these groundfish species, light can alter catch rates. Furthermore, we found possible herding of greenstriped rockfish, and trawl avoidance by arrowtooth flounder, Pacific hake, and longnose skate.