This study examines the effects of a low oxygen environment, in concert with fluctuating temperature and salinity conditions, on the nearshore depth distributions (0-1 m) and flood tide movements of juvenile chinook (Oncorhynchus tshawytscha) and chum (O. keta) salmon. Comparisons are made between an unpolluted and a sewage polluted estuarine intertidal flat in the Fraser River estuary, British Columbia, the polluted area being characterized by the regular occurrence of low dissolved oxygen levels. Results are based on 380 beach seine samples taken between April and June of 1984. In general, chum and chinook salmon of increasing length were captured in increasing depths, though this pattern was modified by seasonal changes in water temperature. Low dissolved oxygen conditions in deeper waters may have been responsible for the presence of larger, and often sluggishly swimming fish in higher oxygenated surface water layers or in shallow waters near the shore. In both areas, the risk of aerial predation was high. On a flood tide, the likelihood of capturing a chinook salmon was reduced as temperatures increased and oxygen levels decreased. A combination of avoidance behaviour and a regularity in the movement patterns of chinook onto the study area in the later stages of the flood tide may account for their rare occurrence in low oxygen concentrations (<6 mg/1) and high temperatures (>20 °C). Fish mortalities were most likely to occur on the ebb tide when fish were forced into waters of low oxygen content by the drainage patterns characteristic of the polluted study area. Though wide ranges in salinity were recorded on both tidal flats, this factor was not strongly correlated to Chinook distributions. However, significantly higher salinity levels in the unpolluted area may account for the greater numbers of chum salmon captured there. An understanding of the influence of estuarine water quality conditions on the distribution of juvenile salmonids may assist in the identification of significant sources of mortality in their early marine life. This knowledge is particularly important in the evaluation of water quality changes as caused by human activity. / Science, Faculty of / Zoology, Department of / Graduate

Low oxygen tolerances of ten fishes were estimated using an original nitrogen cascade design, and reciprocally transformed to express responses as ventilated volume necessary to satisfy minimal oxygen demand (L·mg O2^-1). Values ranged from 0.52 to 5.64 L·mg^-1 and were partitioned into three statistically distinct groups. Eight stream fishes showed moderately high tolerances reflecting metabolic adaptations associated with stream intermittency. Juvenile longear sunfish and two mollies comprised the second group. High tolerance of hypoxia may allow juvenile sunfish to avoid predation, and mollies to survive harsh environmental oxygen regimens. The sheepshead minnow was the most tolerant species of low oxygen, of those examined, explaining its presence in severely hypoxic environments.

The fish community structure of two contrasting estuaries, one with a well developed mangrove forest (Mngazana) and the other without mangroves (Mngazi) was compared. Both the Mngazi and Mngazana estuary fish communities were dominated by marine species, reflecting the importance of these systems as nursery areas for marine fishes. The Mngazi Estuary contained 18% more estuarine fishes in terms of catch per unit effort (CPUE) than the Mngazana Estuary. The reduced tidal influence due to the narrow mouth opening is a possible reason for the heightened CPUE of estuarine species in the Mngazi estuary. The recorded higher diversity of fish species in the Mngazana Estuary when compared with the Mngazi Estuary was attributed to the greater influence of the marine environment due to the wide permanently open mouth, as well as the presence of a variety of habitats in this system. In both estuaries tropical and temperate species were captured, confirming the transitional nature of their biogeographic location which is situated close to the boundary between the subtropical and warm temperate regions of the Southern African coastline. Contrary to previous studies, which recorded seasonal changes in the proportions of tropical and temperate species, the proportions of tropical species remained unchanged at approximately 70% during the January and June sampling occasions. Global warming as a possible reason for the increased dominance of tropical species is discussed. Although several studies in southern Africa have investigated estuarine food web structure, none have compared mangrove and non-mangrove estuaries. In this study, the primary sources of carbon utilised by the fish fauna in the Mngazi and Mngazana estuaries was investigated. The carbon isotopic values of fishes in both estuaries displayed a continuum rather than a tight clustering around particular energy sources. Most detritus feeders of the family Mugilidae (mullets) from both estuaries were relatively more enriched than other fish taxa. The isotopic values of the mullet species suggest a diet derived from relatively enriched carbon sources such as benthic microalgae, the eelgrass Zostera capensis and associated epiphytes. Based on the isotopic values, piscivorous fishes from both estuaries could not be linked to specific prey fish taxa, but clearly the mullet species were not their main food source. The invertebrate feeders that were found in both estuaries showed greater isotopic variations in the Mngazana Estuary than in the Mngazi Estuary, probably reflecting the higher diversity of habitats (carbon sources) and invertebrate prey species in the Mngazana system. Generally the isotopic signatures of fishes from the Mngazi Estuary were more enriched than those from the Mngazana Estuary, thus indicating the possible effect of δ¹³C depleted mangrove derived carbon in the latter system.