Intercohort interactions and settler mortality in a New Zealand reef fish

Background/Question/Methods

In marine reef fishes, intraspecific competition for predator free space can be a main driver of density dependent mortality during settlement.  In species where adults and juveniles overlap significantly in their habitat use, intercohort competition (adults competing with new settlers) may be more intense than intracohort competition (resident juveniles competing with new settlers) due to the higher competitive weights of adults.  Through opportunistic cannibalism, adults may also operate as intraguild predators by competing for a shared resource (space) with their potential prey.  In combination with heterospecific predation, intercohort interactions (competition and predation) may produce second-order or delayed density dependence, where the population growth rate (r) is negatively affected by high adult abundances from previous recruitment years.  In order to examine potential patterns between densities of conspecifics, heterospecifics, and settlers we conducted surveys of a common coastal species in New Zealand, Forsterygion lapillum (the common triplefin).  During the recruitment season, we repeatedly monitored 72 permanent quadrats that were established within two different macroalgal habitats at six different sites (three in an enclosed harbour, three on an exposed coast).  We then used mesocosms to further investigate the simultaneous effects of intercohort competition and predation on settler survival in the presence or absence of a known heterospecific predator.  Finally, using a field manipulation of conspecific density we studied the effects of intercohort competition on the realized spatial niche of juveniles.

Results/Conclusions

Densities of settlers, conspecific, and heterospecific fish were significantly higher in the enclosed harbour than on the open coast.  Heterospecific densities were 5-fold higher in the harbour compared to the open coast; however, heterospecific density had no effect on the abundance of settlers across any of the sites.  Overall, adult densities did not affect the abundance of settlers on the open coast or in the enclosed harbour; however, this was variable over the recruitment season and between sites.  Results from the mesocosm experiment indicate that intercohort predation is low and intercohort competition does not lead to an increase in heterospecific predation.  Preliminary results from our spatial analysis show that despite heavy overlap in the distribution of adults and settlers, intercohort competitive interactions appear to be rare and likely do not affect juvenile spatial use.  Overall, our study suggests that intercohort interactions likely have a minimal effect on yearly variation in recruitment strength and therefore population growth may be largely independent from previous recuitment classes.