- University of Utah
- Queen‘s University
Summary
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Male bluegill sunfish (Lepomis macrochirus) construct nests in densely packed colonies characterized by high breeding synchrony. Females deposit eggs in the nests and males alone provide parental care for eggs and larvae. During the 7-day care period, males do not leave the nests to forage nor do they nourish the young. Parental males do actively defend their broods against predators.
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This study investigated the effects of coloniality on predation. Bluegill brood are attacked by predators moving along the lake substrate and from the water column. Substrate-level predators include a major predator in the system, the snail Viviparous georgianus, and also bullhead, Ictalurus spp. Predation from the water column is primarily by conspecifics (94%) but also by Lepomis gibbosus and the hybrid, L. macrochirus x L. gibbosus. There is little or no predation on parental males.
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Significant differences in brood predation are found among nests in central, peripheral colony and solitary sites. Brood loss at peripheral nests is at least three times that at central nests, and solitary nests experience greater predation than colony nests. These differences are due to effects of nest dispersion rather than to habitat characteristics.
- Bluegill brood predation is reduced through colonial nesting as (1) peripheral nests screen central broods from snails and bullhead, (2) predators can be swamped by the high nesting density, and (3) overlapping defended zones provide cumulative defense against water-column predators and bullhead. Synchrony in breeding augments these anti-predation attributes of colonial nesting, and can also reduce conspecific predation and result in a ‘head-start’ against predators.
- Certain costs to brood survivorship arise from colonial nesting: predation by neighboring males and ripe females, concentration of odor cues which may influence bullhead predation, and possibly fungal transmission between nests.
- Pumpkinseed sunfish (L. gibbosus), which breed concurrently with bluegill, are relatively unsocial nesters. Pumpkinseed do not suffer the same predation pressures. As a result of morphological and behavioral adaptations for feeding, pumpkinseed are able to remove snails from their nests and probably repel bullhead attacks. As snails contribute over 50% of the estimated predation on bluegill, this difference between species is significant.
- Brood predation is proposed as an important selective force for the evolution of colonial nesting in sunfishes. Selection should be mediated through female choice of a nest dispersion which maximizes brood survivorship. Morphological and behavioral preadaptations probably determine the type and degree of brood predation experienced by a species, and hence species-specific selection for patterns of nest dispersion.
Methodology
Synopsis of Reproductive Behavior
Bluegill reproductive behavior in Lake Opinicon is typical for the species (Avila 1973 ; Carlander 1977 ; Gross 1980). Males construct nests in colonies of 10-150 members; nests are built rim to rim and they sometimes assume a hexagonal shape (Barlow 1974), Some males which mate by cuckoldry (Gross 1979; Gross and Charnov 1980), do not construct nests nor show parental care. Gravid females arrive as schools at established colonies of waiting males and spawn for several hours. After spawning, all females return to deeper water. Parental males fan their eggs and respond aggressively to territory intruders. Eggs hatch in approximately 3 days and larvae (immature fry) mature for 4 days before leaving the nests as free-swimming fry. Since spawning is synchronous within a colony, broods of almost all bluegill males are at the same developmental state. After fry leave nests, the males return to deeper water and resume feeding until the next spawning bout. This is usually 3-10 days later. Similar reproductive behavior occurs in pumpkinseed (Miller 1963), but nests are dispersed and breeding synchrony is considerably less.
Study Area
Lake Opinicon (Leeds County, Ontario, Canada) is a 900 ha mesotropbic lake similar to many in the Ontario basin (Keast 1978b). It has a natural bluegill population and a resident fish community of 18 species (Keast and Webb 1966). The primary study site was a homogeneous sandbar 20 m wide and 90 m long, stretching across the mouth of a bay (Birch Bay; see map in Gross and Nowell 1980). Substrate conditions (90% sand, 10% light gravel) and plant cover (5%) are generally uniform along this bar and water temperature does not vary. Water depth on the bar averages 1 m and depth increases to 3 m on the bay side and 10 m on the lake side. Thus the breeding habitat is well delimitated. From four to seven bluegill colonies are simultaneously established on the bar. Collectively they use less than 20% of the total bar area. Between 25 and 80 pumpkinseed nest concurrently in unoccupied regions between colonies. Spawning habitat requirements for pumpkinseed are similar to bluegill. Qualitative observations of predation were made in other areas of the lake, described in Gross and Nowell (1980).
Data Collection and Analysis
The data were collected during the Jun~July breeding months of 1976-1979. Behavioral observations were made from 3-m towers on the shoreline near nesting areas, or with mask and snorkel while kneeling near a colony or swimming along the sandbar. Observer presence caused no apparent disturbance to the fish community (Colgan et al. 1979). Preliminary observation and sampling were made to identify the predators in the system. The potential predators on bluegill brood were identified from approximately 200 h of observation during 1976 and 1977. These were later sampied to verify eggs or larvae in their stomachs. Predator samples were collected randomly using screen funnel traps near colonies, and selectively by using a hand net manipulated by a skin diver.