No abstract

We conducted laboratory experiments to determine relationships between water turbidity, piscivory, fork length, and development of the retina in juvenile walleyes Stizostedion vitreum. Walleyes 85 mm and longer ate a greater weight and number of fathead minnows Pimephales promelas in turbid water than in clear water. Feeding was inhibited at the highest turbidities (100 and 161 nephelometric turbidity units, NTU) in 1‐h feeding trials but not at the highest turbidity (121 NTU) in 4‐h trials. In contrast, walleyes in the shorter length‐group (≤75 mm) fed at approximately the same rate in clear and turbid trials, but consumption was inhibited at the two highest turbidities (100 and 161 NTU) used in 1‐h trials. The tapetum lucidum was present in the retina of walleyes used in all the experiments. Aggregation of photoreceptor cells in the retina into groups of 20 to 30 (to form macroreceptors) begins when walleyes are approximately 60 mm long. Macroreceptors are believed to increase acuity in dim light. In our experiments, the development of macroreceptors was related to increased feeding efficiency in turbid water. The development of scotopic vision early in life permits walleyes to exploit dimly lit environments not used by other predators.

Prior to mid-July, Daphnia pulicaria in yellow perch (Perca flavescens) fry stomachs were smaller than those in plankton collections but after this date mean lengths of D. pulicaria in stomachs and collections were similar indicating an initial selection for small D. pulicaria by the fish. The relation between D. pulicaria body depth and perch mouth gape width indicated that perch fry less than 18 mm long, a length attained in mid-July, could not readily ingest D. pulicaria with body depths greater than 0.7 mm (1.3 mm long). The heterogenic relation between growth in mouth and body size enabled perch fry in West Blue Lake to quickly utilize, as an energy source, the abundant D. pulicaria population.

Retropinna retropinna occurred in two forms in Lake Waahi, North Island, New Zealand. One form was characterized by high mean gill raker counts (c. 28) and low mean vertebral counts (c. 52psimilar to those of non-migratory, lake resident, native smelt in some other lakes of the lower Waikato River system. The other form had low mean gill raker counts (c. 21) and high vertebral counts (c. 60)-similar to migratory smelt at five sites along the lower Waikato River down to its estuary. The lake resident form was shorter but deeper bodied than the migratory form, as indicated by significant differences in mean length and in weight-length regression analyses.The two forms overlapped in time of sexual maturity. As in other native lacustrine smelt populations of the lower Waikato system, the resident form had more but smaller eggs than the migratory form. The egg number to body length regression and the mean egg diameter of migratory Lake Waahi smelt were similar to those of lower Waikato River smelt. Evidence for reproductive separation of the two forms is presented and mechanisms controlling their meristic and other differences are considered.

The feeding ecology of the common smelt, Retropinna retropinna, was determined at several locations on the Waikato River system. The mean size of the dominant prey was found to increase as smelt mouth gape increased. Smelt smaller than 40 mm total length fed mostly on zooplankton. Chironomid larvae, pupae, and adults were the dominant foods of larger fish from most sites, although mysids and amphipods were the major prey in Lake Waahi and the Waikato River estuary. Algae were the dominant food in many smelt but ingestion was probably incidental. Seasonal and annual differences in dominant foods were minor. Common smelt are generalists, capable of feeding on the majority of smaller organisms present in their different environments; however, they may grow faster and achieve a larger adult size when food of optimal size and nutritive value is present.