Koeller: Influence of temperature and effort changes on lobster catches 



65 



ruled out as a cause of the pre-1974 catch 

 cycles. For example, because Halifax harbour 

 sea surface temperatures reflect weather con- 

 ditions in the study area, years with colder 

 sea surface temperatures could also be years 

 with less fishing activity. It is noteworthy that 

 along the eastern shore both pre- and post- 

 1974 catches lag temperatures by 1-2 yr 

 longer than on the south shore. Hudon ( 1994 ) 

 noted that the population along the eastern 

 shore may be growth-limited owing to colder 

 water temperatures than those in adjacent 

 areas, and one would expect this area to re- 

 act more slowly to the more moderate tem- 

 perature increases. 



Catch and effort in smaller statistical dis- 

 tricts along the eastern shore, along with 

 available temperature indices in the area 

 (Fig. 3), reflect the catch patterns of most 

 other lobster fishermen in the northwest 

 Atlantic during this period, i.e. peak catches 

 in the late 1980s and subsequent declines. 

 There was an increase in effort along the 

 eastern shore throughout the period. There 

 is no apparent relation between catches and 

 available temperature indices, although the 

 warmest water and highest catch both oc- 

 curred in the same year ( 1989). 



Port Bickerton reflected the catch and effort pat- 

 tern of the eastern shore as a whole, including an 

 increase in effort throughout most of the period ( Fig. 

 4). Note that the increase in the average daily num- 

 ber of trap hauls leveled off when catches started to 

 decline after 1991. Perhaps fishermen were increas- 

 ing effort in response to increasing lobster catches 

 during the 80s. Pringle and Duggan ( 1984) noted that 

 as many as 25% of the maximum number of traps 

 (eastern shore trap limit is 250) can remain unused 

 in the Nova Scotia lobster fishery. Although the use 

 of catches alone would have overestimated the rate of 

 increase in the population during the 80s and under- 

 estimated the subsequent decline, the differences are 

 small and would not have affected conclusions on an- 

 nual population changes. 



A significant correlation between average daily 

 temperatures and CPTH at Port Bickerton occurred 

 in only 2 of the 8 years for which temperature, catch, 

 and effort information is available at this scale, sug- 

 gesting that short-term, temperature-induced 

 changes in catchability occurred, or were detectable, 

 during those years (Tables 2 and 3). However, a mul- 

 tiple regression of daily catch (dependent variable) 

 versus effort and temperature during these years also 

 indicates that short-term changes in lobster catches 

 were largely determined by effort changes. Cross 



9.5 



9 

 8.5 



8 

 7.5 



7 

 6.5 



6 



Eastern shore 



temperature , 



t700 



Figure 2 



Total annual lobster catch for the eastern shore and south shore of Nova 

 Scotia and average annual sea surface temperatures in Halifax harbor, 

 1950-92. All data series were smoothed with a three year average. 



correlations between total daily catch and effort at 

 Port Bickerton at lags of 0-7 days showed no signifi- 

 cant correlations at lags other than 0, indicating that 

 fishermen's effort was not influenced by catches at 

 short time scales. 



