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Learmonth Bank in northern British Columbia sustains an active trawl fishery that returns large bycatches of deep-sea sponges and corals. To examine effects of biogenic structures on the distribution of fish, we examined nearly 30 km of high-definition imagery from a remotely oper- ated vehicle and documented locations of 2770 scorpaenid fish. The 2 local genera have similar abun- dances, averaging about 1.2 individuals 100 m -2 , but have different spatial abundance patterns: shortspine thornyhead Sebastolobus alascanus are randomly distributed on featureless substrata and their abundance increases with depth. Rockfish Sebastes spp. associate with higher seafloor relief nonrandomly and select for sponges and corals over the inert substrata alone; 95% of the rockfish occurred on 27% of the seafloor surveyed. Sponges (Demospongia and Hexactinellida) were abun- dant on the bedrock and boulders of the bank and adjacent moraine and covered 30 to 55% of the seafloor compared with 1% of the sediment and aggregates of the surrounding basin. The majority of rockfish (80%) occurred with sponges ≥50 cm in height, and even beds of short sponges attracted 4 times as many rockfish than did substrata with no large epifauna. While over half of primnoid corals over 30 cm tall had associated rockfish, less than 2% of the seafloor had large coral, and small coral had no associated rockfish. On the adjacent seafloor with past trawling activity, Primnoa pacifica was 13 times less abundant, and large corals and sponges were rare. Thornyhead abundance doubled but rockfish had a 3-fold reduction in numbers. Our study indicates that degradation of biogenic struc- tures is a long-term detriment to rockfish species and, although the mechanism remains unclear, our data suggest it occurs through the destruction of a habitat that is more effective for shelter than rough inert seafloor.

Shortspine thornyhead and rockfish (Scorpaenidae) distribution in response to substratum, biogenic structures and trawling