Friedlander et al : Sidescan-sonar mapping of benthic trawl marks off Eureka, California 



797 



The general temporal trend in fisheries develop- 

 ment begins with localized fisheries that then expand 

 geographically and to less accessible habitats, such 

 as the deep sea (Deimling and Liss, 1994). In the 

 present study, mean trawl depths have increased over 

 the years as a result of changes in management strat- 

 egies, changes in market demands, and overfishing 

 of nearshore resources. Shortspine thornyheads 

 {Sebastolobus alascaniis) and longspine thornyheads 

 (S. altivelis) are slope rockfish species whose land- 

 ings have increased along the West Coast in recent 

 years (lanelli et al., 1994) whereas English sole has 

 decreased. An export market for thornyheads devel- 

 oped during the 1980s because a similar and highly 

 valued species (S. macrochir) was depleted off Ja- 

 pan (Rogers et al., 1997). As the Japanese market 

 developed, the trawl fishery moved into deeper wa- 



ter following a typical pattern (Deimling and Liss, 

 1994) where longspine thornyheads and larger 

 shortspine thornyheads are most common. Several 

 of the most important species in the trawl catch (e.g. 

 Dover sole and sablefish) segregate at depth by sex, 

 size, and maturity stage. These biological character- 

 istics as well as size and harvest restrictions makes 

 interpretation of changes in the catch of these spe- 

 cies difficult to interpret. 



Implications for management 



The groundfish trawl fishery has experienced declin- 

 ing catches and increasing regulation in recent years. 

 Current management strategies are imposed prima- 

 rily on individual species or among small groups of 

 congeneric species. These strategies do not address 

 the impact to essential fish habitat from fishing ac- 

 tivities and therefore may not be appropriate for the 

 long-term sustainability of these resources. Marine 

 reserves or harvest refugia are an effective manage- 

 ment strategy that can help protect and maintain 

 the complexity and quality offish habitat as well as 

 mitigate the direct effects of fishing (Bohnsack, 1996; 

 Bohnsack and Ault, 1996; Auster and Shackell, 1997; 

 Yoklavich, 1998). A number of studies have noted 

 reduced diversity and abundance of emergent epi- 

 fauna in trawled areas compared with adjacent ar- 

 eas closed to fishing (Bradstock and Gordon, 1983; 

 Auster et al., 1996). The demersal trawl fishery off 

 northwest Australia was thought to be responsible 

 for a significant loss of emergent epifauna and 

 decreased fish productivity (Sainsbury, 1987, 1988), 

 both of which recovered in an area closed to fishing. 

 The establishment of harvest refugia may be the most 

 effective method of reducing the impact of trawls to 

 benthic habitat and subsequently may help to 

 improve the sustainability of the associated fish 

 assemblages. 



Detailed analysis of sidescan-sonar data has al- 

 lowed us to map and quantify trawl marks in rela- 

 tion to fishery and habitat distributions. The combi- 

 nation of geophysical data with fisheries dependent 

 and independent data in a GIS provides the ability 

 to examine the impact of trawling at a scale appro- 

 priate for fisheries management. Evaluation of 

 benthic habitat disturbance caused by fishing activi- 

 ties is important for meeting the mandate of 

 the Sustainable Fisheries Act. Developing indices of 

 benthic habitat disturbance can be valuable in 

 comparing the impact of fishing activities among dif- 

 ferent areas and in establishing baselines for evalu- 

 ating future management strategies such as closed 

 areas. 



