FISHERY BULLETIN: VOL. 80, NO. 4 



nificantly affect the feeding and growth of fish 

 larvae. For these reasons, there is a need for addi- 

 tional detailed descriptions of natural prey so 

 that laboratory studies can be designed to inves- 

 tigate the effects of these prey organisms on 

 larval fish behavior, metabolism, and growth. 



This study was undertaken to examine and 

 compare the food habits and trophic relation- 

 ships of the larvae of five species of marine cot- 

 tids — Myoxocephalus aenaeus, M. octodecemspi- 

 nosus, M. scorpius, Triglops murrayi, and Hemi- 

 tripterus americanus— during winter and early 

 spring when they cooccur at peak abundance in 

 the Damariscotta River estuary, Maine. Data are 

 presented on feeding incidence, diet composi- 

 tion, diet overlap, larval mouth size, and prey 

 size. Trophic patterns are examined with respect 

 to possible interspecific competition and the in- 

 fluence of relative prey abundance and morphol- 

 ogy on foraging. Aspects of the feeding ecology of 

 these larvae are compared with the early feeding 

 ecology of other marine fishes. 



MATERIALS AND METHODS 



Larvae used in diet analyses were collected in 

 surface and bottom tows of a 1 m, 360 nm mesh 

 conical plankton net which was mounted atop a 

 1.3 m wide X 45.7 cm high Blake trawl (a type of 

 beam trawl). Although larvae were collected 

 throughout the Damariscotta River, a drowned 

 river valley opening into the Gulf of Maine and 

 located on the central Maine coast, most speci- 

 mens were captured in the middle basin of the 

 estuary (Laroche 1980). Myoxocephalus (M. ae- 

 naeus, M. octodecemspinosus, and M. scorpius) 

 and T. murrayi larvae were collected, for the 

 most part, on these dates in 1973: 22 January; 6, 

 20, 21 February; and 5, 6, 19, 20 March. From 26 

 to 30% of larvae of the four species used in diet 

 analyses were taken in surface tows and 70 to 

 74% were taken in bottom tows. Larvae of H. 

 americanus were rare in collections; therefore, 

 all specimens collected during the period Janu- 

 ary-April 1972-74 were used in diet analyses. 



Prior to preservation in 10% Formalin 2 in the 

 field, an unquantified amount of MS-222 (tri- 

 caine methanesulfonate) was added to each sam- 

 ple. The sample was gently swirled as the anes- 

 thetic dissolved larvae became inactive. This 



procedure eliminated defecation and/or regurgi- 

 tation subsequent to capture. 



Before removal of the gut, standard length 

 (SL, to nearest 0.1 mm) and upper jaw length 

 (i.e., distance from symphysis to posterior mar- 

 gin of maxillary along the ventral aspect, to 

 nearest 0.01 mm) were measured using an ocular 

 micrometer in a stereomicroscope. Jaw length 

 rather than gape was used as a measure of poten- 

 tial size of the mouth opening because jaw length 

 was not affected by whether the mouth was 

 opened or closed at the time of preservation and 

 could, therefore, be measured more consistently 

 and precisely. Larvae were placed in a cavity of a 

 double-depression glass slide, and the entire gas- 

 trointestinal tract from esophageal sphincter to 

 anus was gently pulled intact from the abdomi- 

 nal cavity. At this time the presence or absence of 

 yolk was noted, and an estimate was made of the 

 quantity of yolk present: one-fourth, one-half, 

 three-fourths of abdominal cavity full or only a 

 remnant remaining. The gut was placed in the 

 other, water-filled cavity of the slide, and the 

 gastric and pyloric regions were teased apart 

 separately using two fine probes. The presence 

 or absence of food items in each of these regions 

 was noted. 



Food items were identified to the lowest taxon 

 possible and counted except for undigestable 

 prey remains, such as setae, and unrecognizable 

 debris. Pseudocalanus eggs, which were prob- 

 ably ingested with the brooding females, and 

 small diatoms (in March samples only) were like- 

 wise not counted. Maximum body width or di- 

 ameter of most food items was measured. 



Larvae were grouped into arbitrarily chosen 

 size intervals, based on overall size distributions, 

 to facilitate intra- and interspecific diet compari- 

 sons. Percent frequency of occurrence (%FO) and 

 percent of total number (%A0 of prey ingested by 

 larvae in each size group were calculated for 

 each food category. An estimate of the relative 

 importance of each food category was obtained 

 by multiplying %FO by %N. Diet overlap was 

 measured using the Schoener index (1970): 



a = 100 [1 - 0.5 i /p x 



1=1 



vjl 



2 Reference to trade names does not imply endorsement by 

 the National Marine Fisheries Service, NOAA. 



where p xi = proportion (percent by number) of 

 food category i in the diet of species x; p yi = pro- 

 portion (percent by number) of food category i in 

 the diet of species y\ and n = the number of food 



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