FISHERY BULLETIN: VOL. 87. NO. 3, 1989 



Accounting for the various sources of sampling 

 bias and for standardizing larval catches facili- 

 tates the calculation of the critical population 

 parameters of abundance and mortality. These 

 parameters are the basis for testing the possible 

 effects of biotic and abiotic variables upon larval 

 fish survival and recruitment and are often used 

 to support or reject a particular hypothesis. The 

 general effect of net avoidance is an increase in 

 estimates of mortality (Clutter and Anraku 

 1968), which in turn can lead to unrealistic con- 

 clusions. 



The interrelationship of mortality and growth 

 parameters has been firmly established for 

 fishes in the marine ecosystem (Beverton and 

 Holt 1957; Ursin 1967; Ware 1975; Shepherd and 

 Gushing 1980; Peterson and Wroblewski 1984; 

 Houde 1987; and others). The underlying as- 

 sumptions for this relationship are that mortal- 

 ity is the result of predation and that predation 

 rates decrease as prey size increases. This sim- 

 plistic model was described as the "cube root 

 rule" (Ursin 1967) where mortality is equal to 

 the cube root of the weight. Peterson and Wrob- 

 leski (1984) expanded this relationship to include 

 gi'owth efficiency and metabolism and estimated 

 the annual mortality rate (M) for larval and adult 

 fishes weighing between 10"'' and 10'^ g as M 

 = (1.92 * year"M weight"-'\ Pauly (1980) 

 noted the rather weak relationship between fish 

 size and mortality (r = 0.38) and found that the 

 inclusion of environmental temperature signifi- 

 cantly improved the fit (r^ = 0.71) of mortality to 

 maximum length and growth rate for 175 fish 

 stocks. The objectives of this study are to deter- 

 mine the changes in larval fish catches during 

 day, night, and twilight hours and provide cor- 

 rection factors to standardize catches for net 

 avoidance. These corrections can significantly 

 change the abundance and mortahty estimates of 

 those species that show a difference in day, 

 night, or twilight catches. Length-dependent 

 larval mortality is estimated from avoidance- 

 corrected abundances for 26 fish taxa and is 

 related to water temperatures. The relation- 

 ships of larval growth, mortality, and water 

 temperature are explored to determine if net 

 avoidance is, in fact, a serious problem as it 

 relates to estimtes of larval mortality. 



METHODS 



Continental shelf and slope waters off the north- 

 east United States have been sampled six to 

 eight times each year since 1977 to provide infor- 



mation on the distribution and abundance of fish 

 eggs and larvae as well as to provide fisheries 

 independent estimates of spawning biomass. A 

 total of 8,312 stations were occupied from 1977 

 through 1984 between Cape Hatteras, NC, and 

 Nova Scotia (Fig. 1). At each station a 61 cm 

 bongo net frame, fitted with 505 |jl and 333 |j, 

 mesh nets and flowmeters, was lowered at 50 

 m/min to within 5 m of the bottom or to 200 m 

 maximum and retrieved at 20 m/min. Ship speed 

 was adjusted to maintain a wire angle of 45 

 degrees; the 505 jjl mesh net was used for 

 ichthyoplankton analysis; and samples were pre- 

 served in 5% formahn. All fish larvae were iden- 

 tified to the lowest taxon possible, enumerated, 

 and measured to the nearest 0. 1 mm standard or 

 notochord length. If more than 50 specimens of a 

 particular taxon were captured in a tow, then 50 

 randomly selected larvae were measured. Water 

 temperature profiles were taken at each station. 

 A detailed account of all shipboard and labora- 

 tory methods, and sampling locations is provided 

 in Sibunka and Silverman (1984). 



Larval catches were standardized (S) to the 

 number under 10 m^ of sea surface by the equa- 

 tion: 



10 * N * D * A'^ *M' 



(1) 



where A^ is the number of larvae in the sample, D 

 is the maximum depth of the tow in meters, A is 

 the area of the mouth of the net, and M is the 

 distance the net was towed in meters deter- 

 mined from the calibrated flowmeter (Smith and 

 Richardson 1977). All analyses followed stan- 

 dardizing of catches to the number of larvae 

 under 10 or 100 m" and rounding of lengths to the 

 nearest mm. 



Each station was assigned to day, night, or 

 twilight hours according to the recorded time at 

 the beginning of the tow. Twilight was desig- 

 nated as one hour before and after both sunrise 

 and sunset while day or night was assigned be- 

 tween the two twilight intervals. 



Catches of all larvae were analyzed by hour of 

 the day to determine if a daily cycle in catchabil- 

 ity could be detected. The assumption in this 

 analysis is that the daily cycle of incident light is 

 the controlling factor for visual detection of the 

 net. The seasonal and latitudinal changes in the 

 duration of daylight hours (e.g., 8.6 hours in 

 winter to 15.4 hours in summer) made it neces- 

 sary, in order to maintain equivalent light 

 regimes across seasons and areas in the analysis, 

 to partition the day into 24 intervals of 10 for 



418 



