Brodziak and Hendnckson: Environmental effects on survey catcfies of Loltgo pealei and ///ex illecebrosus 



11 



October and along a similar cruise track each year. 

 Standardized sui-vey gear, procedures, and the strati- 

 fied random sampling design are described in 

 Azarovitz ( 1981). Offshore survey strata are defined 

 by four depth zones; ranging between 27 m and 366 m. 

 Strata numbers 1-30, 33-40, and 61-76 were in- 

 cluded in our analyses (Fig. 1). Sampling was con- 

 ducted 24 hours a day. Autumn survey data were used 

 because only in autumn are both squid species dis- 

 tributed primarily within the survey sampling area. 

 Survey data during 1963-66 could not be analyzed 

 because catches of squid were not separated by spe- 

 cies during these years. 



At each randomly selected survey station, sampling 

 was performed with a no. 36 Yankee otter trawl 

 rigged with roller gear. Only standard tows, consist- 

 ing of 20-30 minutes duration at a vessel speed of 

 3.5 knots, were included in the analyses. After each 

 tow, the total weight of L. pealei and /. illecebrosus 

 was measured to the nearest 0.1 kg, each species was 

 enumerated, and length-frequency data (mantle 

 length [MLl measured to the nearest cm) were col- 

 lected. The following hydrographic and navigational 

 data were recorded for each station: depth at the start 

 and end of every tow (m), time of tow (h:m, Eastern 

 Standai-d Time ), bottom water temperature ( " C ), and 

 surface water temperature (°C). Towing depth was 

 computed as the average of depths recorded at the 

 start and end of each tow. Surface water tempera- 

 ture was included in this analysis because the effect 

 of surface temperature on the vertical distribution 

 and catchability of these squids was unknown but 

 was anecdotally important and potentially different 

 from the effect of bottom temperature during au- 

 tumn. In general, bottom and surface water tempera- 

 tures within the survey region vary during autumn 

 owing to the transition from a stratified water col- 

 umn, characteristic of summer, to a well-mixed con- 

 dition typical of winter (Bowman, 1977). In particu- 

 lar, the relationship between surface and bottom 

 water temperatures varied on an annual basis dur- 

 ing the 1967-94 autumn surveys. Surface and bot- 

 tom temperatures were not significantly correlated 

 in 8 out of 28 years (29'7c), and exhibited a positive 

 correlation ( p =0.43 ) in 19 out of 28 years (68%) dur- 

 ing 1967-94. 



When an environmental factor was not measured 

 at a survey station, that station was excluded from 

 the association test for that factor. Stations with 

 missing environmental measurements occurred at 

 random, with the exception of the Georges Bank por- 

 tion of the autumn 1990 survey when no bottom tem- 

 perature measurements were made. In total, 7177, 

 7179, 6105, and 6280 stations were available for the 

 association tests of squid catch with depth, time of 



day, bottom temperature, and surface temperature, 

 respectively. 



Within-year associations 



Perry and Smith ( 1994) developed a nonparametric 

 test of association between an environmental factor 

 and the quantity of catch during a stratified random 

 survey. Their method uses the maximum absolute 

 difference between the cumulative distribution func- 

 tion (CDF) of an environmental factor and the catch- 

 weighted CDF of that environmental factor as a test 

 statistic in a randomization procedure to evaluate 

 whether a significant association exists. In particular, 

 the test algorithm is as follows. First, the empirical 

 CDF if) of the environmental factor is computed as 



W, 



ni^=m.—^^^^nh 



(1) 



^,h 



W, 



where t = the value of the environmental factor; 



/; = an index for the survey strata; 



i = an index for the tow in stratum /; ; 



the observed value of the environmen- 

 tal factor from the /"' tow in stratum h ; 

 ^ - the proportion of the sui'vey area in stra- 

 tum li ; 



ri/^ - the number of tows in stratum h ; and 



KxJ- an indicator function with /(.v)=l, when 

 X <t, and /(.v)=0, when .v > t. 



Second, the empirical cumulative distribution of 

 catch as a function of the environmental factor (g) is 

 computed as 



i:',"hys, (2) 



where 



and where y,^,= the catch from the i^^ tow in stra- 

 tum h; 

 y^^ = the mean catch in stratum /? ; and 

 V . = the stratified mean catch. 



^ St 



In Equation 2 above, the quotient y^^/y,, expresses 

 the relative catch under environmental condition .r^^ 

 in comparison with the stratified mean. The maxi- 

 mum absolute value of the difference between f(t) 

 and g(t) over all values of the environmental factor 

 is the observed test statistic (T) where 



T = 



MAX 



y y ^/, I y,h - y^t 



lix,, 



(3) 



