DITTY: ICHTHYOPLANKTON IN NERITIC WATERS 



RESULTS 



Taxonomic Problems 



Larvae of many fishes in the northern Gulf of 

 Mexico are poorly known and taxonomic problems 

 are common, even in some of the most abundant 

 taxa. No attempt was made to identify blennies, 

 gobies, myctophids, synodontids, or cynoglossids to 

 species because of the paucity of literature on lar- 

 val development for these taxa. Little is also known 

 about the taxonomy and morphological development 

 of engraulid larvae. At least five species of engrau- 

 lids are known to occur as adults in the north-central 

 Gulf: Anchoa mitchilli, A. hepsetus, A. lyolepis, A. 

 cubana, Anchoviella perfasciata (Modde and Ross 

 1981), and possibly Engraulis eurystole (Hastings 

 1977). Anchoa mitchilli, A. hepsetus, and A. lyolepis 

 probably account for most of the engraulid larvae 

 collected. Larvae of A. hepsetus and A. mitchilli in 

 the Chesapeake Bay Region can be distinguished 

 from each other primarily on placement of dorsal 

 and anal fins (Manseuti and Hardy 1967), but this 

 character is insufficient to separate reliably the addi- 

 tional species of anchovy that may occur in this area. 

 Separation of menhaden larvae is also difficult. 

 Three species of menhaden are known to occur as 

 adults in this area: Brevoortia smithi (Chandeleur 

 Sound, LA, eastward), B. patronus (Tampa Bay, FL, 

 westward to Veracruz, Mexico) and B. gunteri 

 (Mississippi Sound, MS, westward) (Christmas and 

 Gunter 1960; Springer and Woodburn 1960; Dahl- 

 berg 1970; Turner 1971). Published descriptions are 

 available for laboratory-reared larvae of B. smithi 

 (Houde and Swanson 1975) and B. patronus (Hettler 

 1984) only. Brevoortia gunteri have never been 

 described nor positively identified from the north- 

 ern Gulf. Although the congeners have spawning 

 seasons that reportedly overlap, the center of 

 spawning of B. patronus is apparently off Louisiana 

 between the Mississippi and Atchafalaya River 

 Deltas (Turner 1969; Fore 1970; Christmas and 

 Waller 1975). Since Brevoortia larvae collected dur- 

 ing this study appear similar to that described as 

 B. patronus (Hettler 1984) and because I have recog- 

 nized in subsequent samples (at sizes >7 mm SL) a 

 second morph that could be B. gunteri, all larvae 

 were considered B. patronus. 



Published descriptions of sciaenid larvae are in- 

 adequate to reliably distinguish between small 

 larvae of the species of Menticirrhus (M. ameri- 

 canus, M. littoralis, and M. saxatilis) or between 

 small Cynoscion arenarius and C. nothus. Two types 

 of C. arenarius larvae were recognized primarily on 



the absence (Type A) or presence (Type B) of pig- 

 ment in the dorsal midline immediately above the 

 enlarged melanophore located in the ventral mid- 

 line about midway along the anal fin base. Additional 

 data on the separation of these types are provided 

 in Cowan (1985). Small carangid larvae (<5 mm SL) 

 of certain taxa are also difficult to identify and were 

 referred to a morphological type when a generic or 

 specific epithet could not be assigned. The taxonomy 

 and/or larval development of some of the other 

 monthly dominants (e.g., Lepophidium spp., Ophi- 

 dion spp., Auxis spp., and Ariomma sp.) are poorly 

 understood. 



Hydrography 



Water temperatures between November 1981 and 

 October 1982 ranged from 16°C in January and Feb- 

 ruary to 31° C in June and were below 20 °C from 

 December through February and above 25 °C from 

 May through October. There was little thermal 

 stratification except during the summer, with 

 stratification most pronounced in June (Fig. 2A). 



Salinity stratification was most pronounced from 

 February through August, with little stratification 

 from September through January. Salinities were 

 lowest near the surface, increased with depth, and 

 ranged from <20%o near the surface in February 

 to 32%o in December; salinities near the bottom 

 ranged from 31%o in September to 36%o in the 

 spring and early summer. Salinities near the sur- 

 face steadily decreased from April through July and 

 increased thereafter, whereas those near the bot- 

 tom were comparatively more stable throughout the 

 study period (Fig. 2B). In February, there was a 

 distinct salinity gradient within the upper 6 m of the 

 water column that ranged from 18°/oo at the sur- 

 face to 30°/oo near middepth. In June, two distinct 

 water masses were present with a halocline near 

 middepth. Salinities of these two water masses dif- 

 fered by about 10%o with the less saline waters 

 above middepth (Fig. 2B). Further information on 

 water temperature and salinity variability and the 

 physical processes that affect the hydrography of 

 the study area are provided in Wiseman et al. (1982). 



Seasonal Composition and Abundance 



At least 48 families of fishes were represented in 

 bongo net samples that included 107 taxa, 54 of 

 which were identified to species. About 36,500 lar- 

 vae were collected, with <5% (primarily damaged 

 or yolk-sac larvae) unidentifiable to family. The 

 majority of larvae collected were <5 mm SL except 



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