Age and growth of longfinned eels (Anguilla dieffenbachii) in pastoral and forested streams in the Waikato River basin, and in two hydro-electric lakes in the North Island, New Zealand

Growth rates of New Zealand endemic longfinned eels (Anguilla dieffenbachii) from streams in pasture and indigenous forest, and from two hydroelectric lakes (Lakes Karapiro and Matahina), were estimated by otolith examination. Habitat-specific growth was further investigated with measurement of widths of annual bands in otoliths. Longfinned eels 170-1095 mm in length ranged between 4 and 60 years old (N=252). Eels in pastoral streams grew faster (mean annual length increment ±95% CL = 24 ± 3 mm to 36 ± 7 mm) than eels in streams in indigenous forest (annual length increment 12 ± 2 mm to 15 ± 3 mm). Eels from the hydro-electric lakes had growth rates (annual length increments 19 ± 4 and 19 + 7 mm) similar to eels from pastoral streams. Otoliths of most eels showed annual band widths that indicated growth in several different habitats, corresponding to growth during upstream migration, and limited movement among adult habitats. Estimated age at marketable size (220 g) ranged between 7 and 26 years. The particularly slow growth of longfinned eels in streams in indigenous forest has considerable implications for management. The fast growth rates of eels in hydro-electric lakes provides evidence for the potential of increased eel production by stocking. The probable selective production of female eels in these lakes may be nationally important to allow enhancement of breeding stocks

The role of fish as predators of krill (Euphausia superba) and other pelagic resources in the Southern Ocean

Krill forms an important part of the diet of many Antarctic fish species. An understanding of the role of fish as krill predators in the Southern Ocean is critical to understanding how changes in fish abundance, such as through fishing or environmental change, are likely to impact on the food webs in the region. First attempts to estimate the krill and pelagic food consumption by Antarctic demersal fish in the low Antarctic were made in the late 1970s/early 1980s. Those estimates were constrained by a paucity of biomass estimates and the mostly qualitative nature of food studies. Food consumption estimates were extended to the mesopelagic realm and the high-Antarctic Zone in the late 1980s and early 1990s when these areas were exploited commercially for Electrona carlsbergi. Currently, the best estimates of annual krill consumption by fish are 23 000 000–29 000 000 tonnes of krill and other pelagic prey taken annually by demersal fish in the 1980s in the whole Southern Ocean, and 5 000 000–32 000 000 tonnes taken by mesopelagic fish in the Atlantic sector of the Southern Ocean. It is clear from this review that fish are important predators of krill, in particular the larger myctophids and some channichthyids and nototheniids, and that the importance of krill in fish diets varies substantially both with time and location on various scales, as well as with the availability of alternate prey in the different regions in the Southern Ocean. Ecosystem models therefore need to account for their role. However, several key areas of uncertainty exist, which need to be considered in ecosystem and food-web models for the Southern Ocean. For instance, no robust estimates of food consumption by mesopelagic fish can be provided for the vast areas of the Indian and Pacific Ocean sectors, or for several of the most abundant myctophid species in the Atlantic sector, due to the paucity of relevant studies in these regions. Where biomass estimates do exist, such as in the Atlantic sector, there can be a lack of precision in trawl and hydroacoustic surveys. Studies that adequately capture the seasonal variation in consumption rates and changing importance of krill in the diet of fish are also rare. Furthermore, the effects of large changes in abundance and community structure of fishes brought about by industrial fishing need to be considered when evaluating patterns that have emerged in Southern Ocean ecosystems in the course of the 20th century. As a first step towards a modelling approach to include fish in krill-based food-web models, it is suggested to incorporate data on Champsocephalus gunnari from the western Atlantic sector and Dissostichus mawsoni from the Ross Sea into modelling approaches within the CCAMLR Ecosystem Monitoring Program. Both species are important predators of krill (C. gunnari) and fish (D. mawsoni) in turn form key prey for top predators in each region