Research Document 1998/02

Ajustment for diurnal variation in the catchability of witch flounder (Glyptocaphalus cynoglossus L.) to bottom-trawl surveys in the Gulf of St. Lawrence

By D. Swain and G. Poirier

Abstract

Fishing efficiency for witch flounder during bottom-trawl surveys of the Gulf of St. Lawrence is higher at night than in day (0700≥ time<1900) during August and September surveys but not during January surveys. During the August and September surveys the probability of catching witch was significantly greater at night than in day for all vessel and gear combinations, with the night probability averaging 1.4-1.5 times the day probability in most cases. The difference in average catch rates between night and day was also highly significant in all cases. Night catches by the Lady Hammond using a Western IIA trawl averaged 2.1 times day catches. Night catches by the Alfred Needler averaged about 3.2 times day catches using a Western IIA trawl and 1.6 times day catches using a URI trawl. Patterns in relative abundance were generally similar between unadjusted catch rate series and those adjusted to night catchability. However, the details of these patterns differed between the two series, particularly in the September series between the 1970s when fishing was mostly between 0700 and 1900 and the late 1980s and the 1990s when fishing was conducted 24-h per day.

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