Patterns of distribution and abundance of larval fish in a southern temperate region.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The distribution and abundance of larval fish in the Kaikoura region, on the northeastern coast of the South Island of New Zealand, was investigated over a four year period (1994 - 1997). Spatial and temporal variability were described on both broad and fine scales. Larval fish assemblages at four stations along an offshore transect were sampled fortnightly over an eighteen month period. The abundance and composition of larval fish assemblages were found to vary seasonally. The spawning activity of adults was probably the major determinant of this broad-scale temporal variation in abundance. The timing of spawning activity appeared to be related to the phytoplankton and zooplankton production cycles. The abundance of larval fish at individual offshore stations changed markedly between fortnightly samples. Horizontal movement was considered to be a major cause of this temporal variation. The timing of peaks in larval abundance was relatively constant between years for most species. However, the annual amplitude of these peaks varied considerably for some species. This annual variability in larval supply is likely to have a major impact on the dynamics of the local fish populations. The temporal abundance patterns of the larval stages of most fish species in the Kaikoura region overlapped broadly with those observed for the same species in northeastern New Zealand. The abundance of the larval stages of most species varied with distance from shore. Some species were more abundant further from shore and others more abundant near land. However, there was little evidence to support the generalisation that larvae that are more abundant nearshore hatch from demersal eggs, whereas those that are more widely distributed are derived from pelagic eggs. The alongshore distribution of larvae was investigated at four stations at increasing distances from rocky reefs. Although the larval stages of many species were dispersed at least 6 km offshore, some larval fish, including several that were abundant offshore, appeared to resist alongshore dispersal. These species probably use a combination of active swimming, schooling behaviour and eddies to prevent alongshore dispersal. The fine-scale vertical distribution of larval fish in surface waters was investigated over 24 hr periods. The abundance of the larval stages of most species varied within a 24 hr period. Diel vertical migration was considered to be the major cause of this variation. For some species, the degree of vertical migration appeared to depend on ambient light levels. The horizontal distribution of larval fish in inshore surface waters was strongly influenced by the presence of surface slicks. Larval fish were considerably more abundant within surface slicks than in the surface waters either side of them. This aggregation, together with the shoreward movement of slicks, suggests that surface slicks may transport larval fish towards shore. A new design of light trap was tested in inshore waters. These traps successfully attracted and captured larval fish in inshore habitats. Although the light traps caught a subset of the species taken by plankton nets, they were equally capable of detecting seasonal and lunar phase differences in larval fish abundance. The conclusion of this study was that although local oceanographic processes can directly influence the broad-scale distribution of larval fish, these distributions can be modified markedly by fine-scale processes and the behaviour of larval fish. The ability of the larval stages of many species of fish to adjust their horizontal and vertical position and to maintain a station in suitable habitats results in distributions that are both structured and complex.