Interactions between cultured Fundulus grandis (Pisces: Cyprinodontidae) and the brackish environment: bionomic and economic considerations.

Experimental pond and laboratory studies centered on the growth and feeding responses of gulf killifish Fundulus grandis to varied food sources, salinities, and temperatures. Six 0.1-ha ponds were each stocked on June 21 with 330,000 0.2 g fish/ha and on October 18 with 150,000 0.3 g fish/ha. Trial I compared effects of inorganic fertilization, feeding at 3% body weight daily, and their combination in replicated treatments; Trial II compared fertilization, feeding at 5% body weight once daily and at 2.5% twice daily. Gains (g/day) were higher in fertilized ponds in both trials, but not significantly so. Gain in a pond significantly correlated with gross primary productivity. Harvest biomass and survival means were 710 kg/ha and 70% in Trial I after 80-107 days and 200 kg/ha and 87% in Trial II after 66 days. Low temperatures in Trial II prevented attainment of commercial size. In aquaria fed but without natural food, fish production was 0.33 kg/ha/day or 4-5% of the 5.36 kg/ha/day in similarly fed ponds of Trial I. Outdoor plastic pools, fed and with plankton but without earthen substrates, supported an intermediate production level of 2.80 kg/ha/day. Fertilizing the fed ponds in Trial I elevated fish production by 2.16 kg/ha/day to 7.52 kg/ha/day. Fish in ponds receiving only fertilizer achieved 7.74 kg/ha/day. Greatest positive selectivity was for chironomid larvae, followed by rotifers, calanoid copepods and ostracods. In fed-only treatments overall selection (combined L values) for natural foods and diet diversity was reduced, feeding was oriented to time of feed presence, and increased size variation occurred in 83% of the ponds. Fertilizing ponds resulted in high levels of selectivity and diversity, and decreased size variation in 50% of the ponds. At pond salinities of 2-22 ppt, the zooplankton community was dominated by copepods and rotifers; nekton-epifauna by ostracods; and benthos by chironomid larvae. Fertilization significantly increased primary productivity and abundances of copepods, rotifers, corixids and chironomid larvae. Supplemental feeding significantly increased numbers of polychaete larvae and adults, and bivalves. Of 11 salinities tested, most favorable salinities for hatching eggs and rearing fry were 0-40 and 5-40 ppt, respectively. Of three temperatures compared - 21, 24, 33 C - 24 C produced highest hatch (75%) and a rapid hatch rate (11 days to 50% hatch). Fertilization rather than feeding, and combined growout and hatch ponds decreased projected capital and variable costs for F. grandis production as a live baitfish over previous estimates; resulting in a 104% increase in net revenue/ha from $3571 to $7283. Market size is the major limitation on production potential.