MORSE: CATCHABILITY, GROWTH. AND MORTALITY OF LARVAL FISHES 



daylight, 10 for nighttime, and 2 for each of the 

 twilight times. The number of hours (H) be- 

 tween sunrise and sunset for each station was 

 calculated using latitude and Julian day (Iqbol 

 1983), as follows 



^ .!66M4°f^ovA < 





SCOTIA o 



.'O 





MAINS. / 



!{ CAPE 

 •£/ HATTERAS. 

 §: / N.C. 



Figure 1. — Map of area surveyed, station locations and 

 subareas off the northeast United States. 



H = -7.6393 * arcs[sin(L) * sin(C)/(cos(L) 

 * cos(C))] 



where L = 0.0172(latitude). 



C = 0.3964 - 22.9133 * cos(Z) + 4.1580 



* sin(Z) - 0.3964 * cos(2Z) + 0.5197 



* sin(2Z) - 0.1545 * cos(3Z) + 0.0848 



* sin(3Z), and 



Z = 0.0172 (Julian day -1). 



Twilight hours remained one hotu- before and 

 after sunrise and sunset and were partitioned 

 into hour intervals 5-6 and 17-18, respectively, 

 and accounted for 4 of the 24 h day. The remain- 

 ing daylight hours (H - 2), where H is time 

 from sunrise to sunset, were partitioned into 10 

 equal intervals (i.e., hours 7-16). Nighttime 

 hours (24 - (H + 2)) were also partitioned into 

 10 intervals corresponding to hours 0—4 and 19- 

 23. The time interval for a daylight "hour" 

 ranged from about 52 minutes in winter to 92 

 minutes in summer. Each station was assigned 

 an "hour" interval and analysis was performed 

 during this interval. 



The occurrence of tows that did not contain 

 larvae or that did not contain the particular 

 taxon of interest presented a special problem in 

 day-versus night-catch analysis. The absence of 

 larvae might have occurred because there were 

 no larvae within the path of the net or because of 

 net avoidance by the larvae. This being the case, 

 all stations in cruises that contained at least one 

 occurrence of the taxon being investigated, ex- 

 cept where noted, were included in calculations 

 of mean catch per 10 m". The result of including 

 these zero tows was a reduction in the mean 

 catch, but a stability in the catch ratios. In addi- 

 tion, the survey area was divided into four sub- 

 areas: Middle Atlantic Bight (MAB), Southern 

 New England (SNE), Georges Bank (GB), and 

 Gulf of Maine (GOM) (Fig. 1). The subareas were 

 used to stratify the entire survey area during 

 analysis for those taxa that spawn in a limited 

 area. For example, if no larvae of a particular 

 species were caught in GOM subarea then all 

 GOM stations were ignored for the analysis of 

 that species. The calculation of the mean catch 

 and its variance using zero tows followed the 

 methods of Pennington (1983) for the Delta dis- 

 tribution of catch frequencies. Ratios of all 

 larvae for day, night, and twilight were based on 

 mean catch per 10 m", and ratios for millimeter 

 length increments were based on mean catch per 



419 



