Summary
Stamps (1978) suggested that the type and extent of parental care can influence the behavioural ontogeny of the offspring. Largemouth bass (Micropterus salmoides) males guard the fry for up to a month after they leave the nest while rock bass (Ambloplites rupestris) males guard the fry only until they leave the nest. In laboratory experiments, naive largemouth fry displayed reduced predator-avoidance responses to large and small predators during their first 3 weeks of free-swimming. Rock bass fry, on the other hand, avoided predators throughout the study period. From 6 weeks (largemouth) and 5 weeks (rock bass) of age the fry showed a significantly greater response to the large predator than the small predator. The responses of laboratory-reared largemouth bass fry to the small predator changed from avoidance to association as the fry grew. The size of the fry relative to predator size was found to be important.
Methodology
The study was carried out at the Queen's University Biological Field Station located on Lake Opinicon 40 km north-east of Kingston, Ontario. Eleuthero embryos (Balon 1975) were collected from largemouth and rock bass nests and brought into the laboratory for rearing. The term fry refers to fish that have commenced free swimming and are less than 1 year old, with age being calculated from the onset of free-swimming. Four holding tanks with 25-50 fry in each were maintained for each species. Fry were fed cultured wild plankton three times per day. All tanks were supplied with a continuous flow of lake water and a natural photoperiod was maintained. A thin layer of gravel covered the bottom of each tank.
Fry were tested from June to August in 1980 and 1981. Two sizes of experimental tanks were used: four large tanks measuring 76 x 31 x 25 cm and four small tanks 50 x 30 x 25 cm. The large tanks were used during the latter part of 1980 to provide more space for the larger fry to avoid the predators. Analysis indicated that no differences existed in the times spent in the grids (see below) so in 1981 only the small tanks were used. The experimental tanks were divided by glass partitions into three areas. The central test area was divided into three equal parts (four in the large tanks) by black lines drawn on the outside of the glass. The grids in this area were designated as the P grid (grid next to predator), the F grid (grid furthest from predator), and the N grid (the grid(s) between the P and F grids). Two sizes of predator were used: a yearling conspecific and a 3-year-old rock bass. Preliminary experiments conducted in 1979 indicated that the response of the fry to a yearling conspecific predator changed over the summer as the fry grew. To document this change, a yearling conspecific was used as the small predator. Three year-old rock bass were used as the large predator because they adapted to handling and laboratory routine well and are piscivorous in the lake (Keast 1978). Also, since adult male largemouth bass guard the largemouth fry for a period of time, it is possible that largemouth fry might display a tendency to approach any adult largemouth used as a predator. An approach response by fry to their parents has been recorded in some cichlid fry (Keenleyside 1979). For this reason largemouth bass were not used as predators. Both types of predator were presented with fry bi-weekly in an open tank to determine if they would prey upon the fry. These fry were not retested. The sizes of the yearling conspecific predators ranged from 42 to 61 mm for rock bass and from 81 to 103 mm for largemouth bass. The sizes of the large rock bass predators ranged from 135 to 155 mm.