Diet, nutrition, and growth in the temperate rocky reef fish Girella tricuspidata (Girellidae)

Abstract

This thesis investigated the relationship between nutritional ecology and growth in the marine reef fish Girella tricuspidata (Girellidae) found in northern New Zealand and temperate eastern Australia. The aim was to establish how diet, nutrition, and temperature affect the demography of the species/population, and in particular to test two hypotheses. The Temperature-Constraint Hypothesis predicts that digestion of algal foods in marine herbivorous fish is constrained by temperature at higher latitudes, and therefore restricts growth, whereas the Temperature-Size Rule predicts that temperature determines growth rate. This thesis investigated the effects of changes in diet and nutrition on growth and longevity between two populations of Girella tricuspidata, and how diet and nutrient intake were scheduled across the year in relation to growth and reproduction. The aim was also to identify the strategies omnivorous fishes on a mainly herbivorous diet employ to survive on a diet considered nutritionally poor and difficult to digest. Spatial differences in growth between populations were analysed to examine the effects of nutrition and temperature on growth. Two populations (coastal and offshore, about 50 km apart) were sampled in the Outer Hauraki Gulf in north-eastern New Zealand. Diet analysis of stomach content samples revealed an omnivorous diet, with fish ingesting a wide array of food items. The diet consisted mainly of small epiphytic Rhodophyta, Abroteia suborbicularis in particular, complemented by a considerable amount of animal material. Epiphytes on Carpophyllum maschalocarpum varied in abundance seasonally, but the pattern differed between two coastal sampling locations, indicating that epiphytes are available throughout the year. Ulva species (Chlorophyta), which are ususally considered to be the preferred food, were also ingested but were not a dominant dietary item. Seasonal changes in diet compositions were mainly due to the seasonal appearance of salps, which formed a major part of the diet between spring and autumn. Salps appeared slightly earlier in stomach contents of fish offshore, where abundances peaked earlier in the season due to the hydrology of the Hauraki Gulf. When salps were not available fish increased their intake of other animal matter such as crustaceans. Nutrient analysis (carbon, nitrogen, lipid, ash) of stomach contents revealed that diet items differed in nutrient compositions and that diet composition varied between locations and seasons. However G. tricuspidata mixed diet items so that the nutrient composition remained nearly constant throughout the year and was similar for both populations. The condition factor based on gutted weight remained nearly constant throughout the year, but increased slightly in spring based on the total weight, a pattern associated with gonad development. Spawning peaked in December. Nutrient demand was expected to increase during spawning time in spring, but relative gut content mass and nutrient composition remained constant in both populations, indicating consistent nutrient intake throughout the year. Intake of salps, which contain high lipid levels needed for reproduction, increased during spawning time. Relative gut length varied between seasons in both populations, and coastal fish had longer guts in relation to body length than offshore fish. G. tricuspidata is a long-lived temperate reef fish that displays an asymptotic growth pattern. Annual increments in sagittal otoliths revealed 54 years of age for the oldest fish, an offshore specimen, representing the maximum-recorded age reported for G. tricuspidata. The oldest fish caught from the coastal population was 44 years. Otolith chronologies showed that increment widths reflected increased growth during warmer years, while colder years resulted in increment widths narrower than average. Growth increment width indices correlated strongly with summer sea surface temperatures. There was a significant difference in growth rate between the coastal and offshore populations, but not between sexes. Calculations of the reparameterized von Bertalanffy growth function parameters showed that coastal fish grew faster as juveniles. Population growth curves crossed at about seven years. Coastal fish reached their adult size at about 13 years and offshore fish at 18 years. Coastal fish had smaller mean adult sizes (291.5 mm SL) than offshore fish (326.3 mm SL). The determining factor was most likely microhabitat. Juvenile coastal fish spend their first two years exclusively inside Whangateau Harbour before moving to the open coast, and adult fish also spend time inside the Harbour. Summer sea surface temperatures are about 2.9°C warmer inside Whangateau Harbour than along the coast and at the offshore location. This study suggests that G. tricuspidata selectively feed on protein-rich algae and also employ a complementary feeding strategy gaining the majority of their energy from epiphytic algae. Epiphytic algae are complemented with animal matter rich in protein and lipid. Data on resource availability, nutrition, and digestion was inconsistent with the Temperature-Constraint Hypothesis. Rather, temperature variation influenced growth variation over the spatial scale of the study, and temperature differences between habitats were most likely the factor driving observed growth variation between the two populations, thereby supporting the Temperature-Size Rule.