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The dietary phosphorus (P) requirement for large fish is difficult to estimate because of insensitivities of known P status indicators. We examined dietary P requirement of large rainbow trout (mean body weight 278 g) using recently identified P-responsive genes (mRNA abundances) as well as conventional serum P and bone P. Fish were fed six diets (varied P contents), and the tissues of intestine, pyloric caeca (PC), kidney, serum and bone were collected at varying time intervals. Serum P responded clearly to dietary P by day 2, but the estimated P requirement based on this variable changed as feeding duration continued. Bone P did not respond clearly until week 7. Among P-responsive genes studied, Na/Pi cotransporter in PC (PC-NaPi) was the most sensitive, and responded in 2 days. Fish-to-fish (within treatment) variance was larger in mRNA than in serum P and bone P levels. Estimated dietary P requirements (%P in dry diet) were 0.45 (based on serum P), 0.45 (based on bone P), 0.36 (based on PC-NaPi), 0.33 (based on intestinal NaPi), 0.71 (based on renal NaPi), and 0.33 (based on mitochondrial Pi carrier). This study is the first to evaluate the potential of genomic approaches in determining nutrient requirements of fish.

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Reproduction by stocked Lake Trout Salvelinus namaycush is generally estimated as the relative abundance of fry, that is, catch per unit effort in emergent fry traps and in beam trawls, but these estimates have high variance due to spatially heterogeneous distributions of fry. We used calcein, which produces a fluorescent mark in calcified structures, to batch‐mark fry and generate a mark–recapture estimate of fry abundance on a small, shallow spawning reef. Eggs collected from feral Lake Trout in Lake Champlain, Vermont were reared at ambient lake temperatures, and fry were marked 7 d after hatching. Fry were immersed in a salt solution for osmotic induction and then placed for 4 min in a calcein solution. After marking, 18,000 fry were released on a spawning reef, and 2,000 fry were maintained in the hatchery. Wild‐caught fry and hatchery fry were checked for marks every 2–9 d. Mark clarity was highest in the mandible and tail rays. Marks may have faded, but they did not disappear: marks were visible in the mandible in 100% of hatchery fry after 68 d. An average of 37% of wild‐caught fry had marks, yielding a Chapman population estimate (±SD) of 47,486±2,301. The mark–recapture estimate was within the range of fry abundance estimated over 6 years based on egg density data and estimates of hatching success but was substantially higher than estimated for the same year‐class. This work supports prior estimates of fry abundance and provides a potential method for assessing fry abundance on deep reefs and the success of fry stocking. Received June 4, 2013; accepted November 20, 2013

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