62 



Abstract.— Correlation analysis was 

 used to investigate how effort and tem- 

 perature changes influence lobster 

 catches at different spatial and tempo- 

 ral scales and how they may affect the 

 use of catch statistics in stock assess- 

 ments. At the largest scales examined 

 (Atlantic coast of Nova Scotia, 50 yr), a 

 significant correlation between catches 

 and temperatures at short lags (0-3 yr) 

 prior to 1974 suggests that catches 

 were driven by temperature-induced 

 changes in growth or lobster activity. 

 However, changes in effort could not be 

 ruled out as a cause of the observed 

 cycles because sea surface temperatures 

 may reflect weather conditions and the 

 "fishability" of the grounds. Longer lags 

 (6-8 yr) after 1974 are consistent with 

 increased larval survival due to sharply 

 rising temperatures and the "recruit- 

 ment pulse" of the late 1980s. There 

 was no clear relation between tempera- 

 ture and catches at intermediate scales 

 (statistical districts. 10 yr), but effort 

 changes indicate that catches alone do 

 not accurately reflect changes in lob- 

 ster abundance. At the smallest scale 

 examined (distances between ports, 

 days) correlations of both temperature 

 and effort with catch in areas with simi- 

 lar coastal topographies indicated that 

 the correlation between temperature and 

 catch was not causative, i.e. changes in 

 effort, were driven by wind events that 

 were also influencing water tempera- 

 tures. The results indicate that effort 

 changes must be considered at all 

 scales in stock assessments, but they 

 become increasingly important at the 

 smallest (i.e. <100 km. within years) 

 scales. They also indicate that signifi- 

 cant correlations between lobster catches 

 and environmental parameters must be 

 interpreted cautiously. 



Influence of temperature and effort on lobster 

 catches at different temporal and spatial scales 

 and the implications for stock assessments 



Peter Koeller 



Department of Fisheries and Oceans 

 Invertebrate Fisheries Division 

 Bedford Institute of Oceanography 

 P.O. Box 1006, Dartmouth 

 Nova Scotia, Canada B2Y 4A2 

 E-mail address koellerpigmardfo-mpogcca 



Manuscript accepted 19 March 1998. 

 Fish. Bull. 97:62-70(19991. 



In the absence of suitable effort 

 data, long-term, large-scale changes 

 in American lobster (Homarus 

 americanus) populations are gener- 

 ally assessed by using summarized 

 catch statistics (e.g. Harding et al, 

 1983; Pezzack, 1992; Drinkwater et 

 al, 1991, 1996), under the assump- 

 tion that catch trends reflect abun- 

 dance changes. Two observations 

 support this assumption. First, high 

 exploitation rates in most areas 

 probably remove the majority of 

 animals recruiting to the fishery in 

 any year (Miller et al, 1985). Sec- 

 ond, similar catch trends over large 

 areas of the Northwest Atlantic sug- 

 gest a similar population response 

 to a common environmental influ- 

 ence (Pezzack, 1992). Given that 

 this assumption is correct at these 

 large scales, it is of fundamental 

 interest in survey design and data 

 aggregation decisions to determine 

 the spatial and temporal scales at 

 which this assumption holds. For 

 example, can the catch statistic 

 alone be used to assess changes in 

 lobster populations between, say, 

 adjacent ports within seasons? 



To understand the spatial and 

 temporal variability of lobster 

 catches on the Atlantic coast of 

 Nova Scotia, Canada, and the un- 

 derlying causes of this variability, 1 

 examined the influence of effort and 

 an important environmental factor 

 (temperature) on commercial lob- 

 ster catches at different spatial and 



temporal scales. Temperature can 

 affect lobster catches in the short 

 term by altering the availability of 

 lobsters to traps, i.e. by changing 

 lobster behavior on the grounds 

 (McCleese and Wilder, 1958) or by 

 increasing growth of prerecruits to 

 commercial sizes. In the long term 

 it could also affect population 

 growth by influencing survival of 

 lar'v'ae or juvenile stages (Aiken and 

 Waddy 1986). Changes in effort af- 

 fect lobster catches directly but how 

 these changes occur is complex. In 

 a lucrative fishery with a relatively 

 short (approx. 60 d) open season and 

 restrictions on the number of traps, 

 fishermen tend to take the maxi- 

 mum number of lobsters possible 

 with their gear. Consequently, 

 changes in effort are more likely to 

 be due to factors that decrease ef- 

 fort. For example, fishermen could 

 decrease the number of traps fished 

 and frequency of hauls when catches 

 are too low to make fishing worth- 

 while. Because the fishery is con- 

 ducted from small boats, adverse 

 weather conditions will also de- 

 crease effort. Conversely, fishermen 

 may respond to increasing catch 

 rates or good weather by fishing 

 harder. Because catches alone are 

 often used as an indication of stock 

 status, it is important to under- 

 stand how changes in effort may be 

 influencing catches at the different 

 spatial and temporal scales at 

 which assessments are conducted. 



