Understanding the spatial structure of populations including variability in reproductive biology among subpopulations is a key objective in fisheries ecology. The aim of the present study was to identify the small scale (<10 km) population structure of blue cod (Parapercis colias) and sea perch (Helicolenus percoides) along the Otago coast and Fiordland to understand key processes that may have resulted in important demographic structure among subpopulations.
Here, comparisons of size structure of blue cod among coastal and offshore regions of the Otago coast were used to estimate the biological consequences of size truncation. Populations strongly truncated towards smaller individuals were observed in coastal regions of likely higher fishing pressure. These truncated populations likely produced several orders of magnitude fewer eggs per capita than those with larger, older mature individuals. The result was a highly structured population with areas of low reproductive output and areas of higher reproductive output in close proximity, or a source-sink population.
Similarly, the spatial structure of sea perch populations was investigated over larger scales. Here, intraspecific differences in morphology, growth, and isotopic signature were found among sea perch populations from Otago shelf compared to inner and outer coast regions in Fiordland. These results suggested that sea perch formed discrete subpopulations or stocks of fish in each region. The spatial population structure was most likely linked to habitat type, with distinct morphology, growth and isotopic signature associated with inner fjord and outer coastal sites.
Further, maternal characteristics of sea perch from the Otago shelf including age, length, weight and condition were used to predict oil globule volume, notochord length and growth rate of cohorts of larvae. The results indicated that larger, older females produced offspring with larger energetic reserves, and likely better survivorship, than offspring from smaller, younger females.
The investigations demonstrated important links between size and age structure and spatial population structure of two heavily exploited coastal fishes. The results have important implications for understanding population dynamics and the likely response of structured populations to exploitation or spatial management
