Biogeographic variation in demographic rates of the yellow-eyed mullet, Aldrichetta forsteri (Mugilidae) across New Zealand

Abstract

A central question of ecology and biogeography concerns the role of environmental- and population processes as determinants of species distribution and abundance. Members of the Mugilidae, a family of pelagic fish species, are consistently among the most abundant and widespread species found in New Zealand. Using a range of field- and laboratory- based techniques, I explore patterns of distribution, abundance, and life history characteristics of the species Aldrichetta forsteri (Yellow-eyed mullet) from 13 estuaries/harbours in New Zealand. I examine potential causes and consequences of these patterns within the contexts of ecology and biogeography. In Chapter 1, I review the biogeographical background associated with a marine species distribution and abundance, with regard to historical processes, and the associated implications of establishing patterns of distributions within the marine realm. While these large scale processes acting over historical timescales offer one aspect of the species Yellow-eyed mullet, (Aldrichetta forsteri), biogeography, I address the more relevant question of understanding the patterns and sources of variation operating at a local scale for the interpretation of this species' present day biogeographic distribution. In Chapter 2, I explore these spatial patterns of abundance for the species A. forsteri by relating patterns of abundance to patterns of abiotic (e.g. environmental), and biotic (e.g. density-dependent interactions, dispersal) factor variation found in estuaries/harbours from around New Zealand. I relate patterns of abundance to biogeographical caveats associated with a species' distribution as described in works by Brown (e.g. hot spots, centre of origin; 1984, 1995), Hutchinson (e.g. multidimensional niche; 1957, 1958), and Fretwell (e.g. ideal-free distribution; 1972). Estimates of abundance, local population densities, sex-ratios, and size structure, obtained through gill netting at each of the estuaries, were found to largely exhibit spatial variation in response to varying local environmental conditions. Spatial variation in size, sex-ratios, and abundance all exhibited variation in accordance with a latitudinal cline, whereby, sea surface temperature best explained the patterns observed. The findings support the idea that local demographic levels are influenced by the extent to which the local environment meets the requirements of the individual. Often in many of the cases the population parameters are coupled with an environmental variable along a latitudinal gradient, whereby, localities offering the most optimal conditions experience the most 'healthy' populations of A. forsteri. This analysis provides an initial insight into the variability within and between estuarine ecosystems, and although this investigation focused on the species A. forsteri, results may be applicable to other estuarine species. In Chapter 3, I employ a set of approaches commonly used by fisheries scientists, to further examine spatial variation in life-history attributes of growth and mortality in the species A. forsteri. Specimens collected from the field (n=511) had both sagittal otoliths removed, which were subsequently sectioned and mounted upon glass slides for age estimation. A sub sample of these otoliths had daily increments validated by immersion in Alizarin Red; while annual rings were validated by confirming ~365 daily increments present between "annual" bands. Size-at-age data had both linear, and non linear (von Bertalanffy), growth models fitted while incorporating a statistical methodology (Akaike Information Criterion) for the selection of the model that 'best' described the data. Growth rates of A. forsteri, as best described by the linear growth models, again exhibited spatial variation in accordance with environmental variables associated with each of the sites. Latitudinal clines in growth rates were either very weak, or absent from the size-at-age data for A. forsteri, while males tended to experience greater growth rates than their female counterparts, possibly as a result from the associated costs of reproduction incurred by the female. The linear growth pattern of A. forsteri, when coupled with the species mortality rate, indicated a growth-mortality relationship of locations exhibiting the highest growth rates so to experiencing the highest mortality rates. If the relationship found truly reflects biological parameters within populations of A. forsteri, then this apparent relationship contradicts the views held by many ecologists regarding an inverse relationship between growth and mortality. The mortality rate values (> 67% annual mortality) obtained from the catch-analysis method are similar to the best estimate (66%) specified by Taylor & Paul (1998). The high adult mortality rate is consistent with its other life-history traits of rapid growth, and high reproductive output. Spatial variation in both growth and mortality rates varied according to the combination, and level of local abiotic and biotic variables that occurred at each geographic location, offering support to Fretwell's (1972) theory of an 'ideal free' distribution, accordingly distributed to how well local variables meet the requirements of the individual. Although not commonly associated with the life history traits of a larger sized species (e.g. linear growth, & positive growth-mortality relationship), it is possible this species may exhibit biological parameters that are more often associated with small, short lived, and r- selected species, and therefore may not conform to life-history predictions associated with a larger size. Finally, I conclude my thesis with a synthesis of my findings within the context of ecology and biogeographic theory. Overall, results suggest that both environmental and population processes are likely to interact to determine patterns of abundance and distribution of the species A. forsteri. The fact that spatially separate populations exhibited variation in demographic parameters in relation to primarily locally driven processes offers evidence for several biogeographic mechanisms. The idea of Brown's 'hot spots' and environmental gradients, or Fretwell's 'ideal free' distributions were all applicable to several of the life-history parameters (e.g. growth rate, mortality rate), and population parameters (e.g. abundance, average size, year class, sex-ratios) for the species A. forsteri. Such an occurrence re-iterates the highly variable nature of these brackish water ecosystems, and offers an insight into the level of complexity and scale needed to be employed when dealing with these environments, and their associated fauna in terms of restoration, mitigation and fisheries management.