Catchability, Growth, and Mortality of Larval Fishes 



Wallace W. Morse 



ABSTRACT: The catchability of fish lar\ae with 

 a 61 cm bongo net was determined from analysis of 

 day, night, and twilight samples from 8,312 stations 

 made off the northeast United States during 1977- 

 84. Night catches exceeded day catches by 629( and 

 twilight exceeded day catches by 44%. Catchability 

 by year and month revealed some variations; 

 however, night catches still dominated. The daily 

 cycle in catchability of all lar\ae showed that the 

 maximum catch occurred at approximately 0200, 

 and was about 2.5 times the minimum catches at 

 1700. Changes in catchability with water column 

 depth reflected changes in species composition of 

 the catches. 



Analysis of .36 taxa revealed 1 1 significant differ- 

 ences in dayrnight, day:twilight, and night:twilight 

 catch ratios. Length-dependent catchability and 

 correction factors were determined for 26 species. 

 Corrected length frequencies were used to calculate 

 length-dependent mortalities which are shown to be 

 positively correlated with water temperature. 

 Larval growth rates were also found to be tempera- 

 ture dependent and, by incorporating a length- 

 weight coefficient, larval length was converted to 

 age. Age-frequencies were used to calculate daily 

 lanal mortality and, for most species, the ratio of 

 mortality rate to growth rate was approximately 

 0.8. Those species with ratios at or over 1.0, i.e., 

 bluefish, Pomatomus saltatrix; Sebastes spp.; 

 Atlantic mackerel, Scomber scombrus; cunner, 

 Tautogolabrus adspersus; and, to some extent, 

 haddock, Melanogrammus aeglefinus, probably 

 exhibit significant net avoidance. 



Surveys of ichthyoplankton in the marine envi- 

 ronment have been an integi-al part of fisheries 

 science for nearly a century. Today large marine 

 ecosystems are routinely monitored for both 

 physical and biological parameters to study 

 multispecies interactions during the early life 

 history of fishes (Sherman 1986). One of the 

 major goals of large marine ecosystem surveys is 

 to monitor the inter- and intra-annual changes in 

 larval fish abundance and mortality and their 

 relationship to recruitment of fishable biomass. 



Wallace W. Morse, National Marine Fisheries Service, 

 Northeast Fisheries Center, Sandy Hook Laboratory, High- 

 lands. New Jersey 07732. 



Manuscript accepted March 1989. 

 Fishery Bulletin, U.S. 87: 417-446. 



Demonstrating the relationship between larval 

 abundance and recruitment has remained elusive 

 owing, in part, to the complexities of the interac- 

 tions of the physical and biological factors that 

 affect survival of fish during the first year of life. 

 However, a number of theories have been pro- 

 posed that attempt to link larval fish survival 

 and, by inference, recruitment with their food 

 supply. These include the "critical period" 

 theory (Hjort 1914; May 1974), the "match- 

 mismatch" theory (Gushing 1975), and the mixed 

 layer stabihty hypothesis (Lasker 1981). 



A requisite condition for the testing of these 

 and other theories using broadscale ichthyo- 

 plankton surveys is what Zweifel and Smith 

 (1981) call an "effective sampler size". This 

 involves accounting for the "effect of environ- 

 mental and behavioral factors on the content of 

 the samples or collections". The objective of 

 accounting for these factors is to standardize 

 the sampling gear by applying correction fac- 

 tors in order to minimize sampling variability 

 and to make samples comparable (Smith and 

 Richardson 1977). A key consideration and a 

 possible serious source of bias in larval fish 

 collections is net avoidance (Clutter and Anraku 

 1968). Fish larvae can avoid capture by swim- 

 ming out of the path of an approaching net or by 

 migrating below the maximum depth sampled 

 by the net. If detection of an approaching net 

 by larvae is visual, changes in light intensity or 

 net coloration will alter catchability. The 

 magnitude of visually cued avoidance is indi- 

 cated by the variation in day versus night 

 catches. 



Differences in day versus night catches for 

 numerous species have been reported for the 

 past 60 years (e.g., Johansen 1925; Russell 1926; 

 Ahlstrom 1954; Bridger 1956; Miller et al. 1963; 

 Lenarz 1973). In most cases night catches exceed 

 day catches. But there is evidence that gear con- 

 figuration and towing speed affect the ratio of 

 day-to-night catches (Bridger 1956; Clutter and 

 Anraku 1968). For example. Miller et al. (1963) 

 attributed the lack of difference in day-to-night 

 catches of haddock. Melanogrammus aegle- 

 finus, to the high speed (4 m/s) gear they used. 



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