Niche partitioning in the Fiordland wrasse guild

Abstract

Investigating the parameters of niche space for marine organisms is useful for identifying their place in the food web and understanding community dynamics. The range of biological and physical gradients found along the fiord axis in Fiordland, NZ offer a unique setting that is well suited for defining aspects of niche space. The purpose of this thesis was to define the characteristics of niche space of four common species of wrasse found in Fiordland, including spotties (Notolabrus celidotus), scarlet wrasse (Pseudolabrus miles), banded wrasse (Notolabrus fucicola) and girdled wrasse (Notolabrus cinctus). Fish were sampled from three paired inner and outer fiord sites in Doubtful Sound, Thompson/Bradshaw Sound and Dusky Sound. Gut contents analysis, stable isotope analysis and morphological analysis of facial features associated with foraging strategy were applied in an effort to characterize diet as an indicator of biological niche space. Fiordland-wide SCUBA surveys of abundance and distribution were correlated with both physical and biological gradients to observe evidence of habitat partitioning. Finally, age and growth analysis was evaluated to assess evidence of trade-offs in life history strategy. Based on these indicators, spotties were found to occupy a significantly different niche space than scarlet wrasse, banded wrasse and girdled wrasse. High niche overlap was found among these other species, with some differences in prey field and microhabitat partitioning. This pattern is mainly attributed to the fact that spotties were observed to be a highly plastic species (generalist), exploiting a wide range of resources over the length of the fiord axis. Conversely, other wrasse species exploited a relatively narrow range of resources (specialists), preferring highly productive kelp forest habitat near the outer coast. Spotties were found to partition niche space between inner and outer fiord environments, consuming a less nutritious diet in the inner fiord by exploiting both terrestrial sources of carbon and alternate trophic levels. In conclusion, the role of wrasse species as pervasive, abundant invertebrate predators in the Fiordland ecosystem indicates they likely play an important role in maintaining ecosystem structure. Though niche partitioning occurred among species and over spatial gradients, outer coast areas appeared to support the most valuable, high quality habitat for Fiordland wrasse species.