Physiological ecology of juvenile Chinook salmon (Oncorhynchus tshawytscha) rearing in fluctuating salinity environments

Graduation date: 2005Estuaries provide juvenile salmonids with highly productive feeding grounds,\ud refugia from tidal fluctuations and predators, and acclimation areas for smoltification.\ud However, these dynamic, fluctuating salinity environments may also be physiologically\ud stressful to growing juvenile fish. In order to evaluate the costs and benefits of estuarine\ud marshes to juvenile Chinook salmon, I observed habitat use, diet, and growth of fish in\ud the Nehalem Estuary on the Oregon coast. I also examined physiological costs associated\ud with salmon living in fluctuating salinities and growth rates in laboratory experiments.\ud I collected growth, diet and osmoregulation information from juvenile Chinook\ud salmon in three tidal marsh sites in the Nehalem Bay and from juveniles in the Nehalem\ud River. Stomach contents indicated that a high proportion of the diet is derived from\ud terrestrial prey. These allochthonous prey resources likely become available during the\ud flood stages of tidal cycles when drift, emergent and terrestrial insects would become\ud available from the grasses surrounding the water. This field study confirmed that\ud juvenile Chinook salmon utilized fluctuating salinity habitats to feed on a wide range of\ud items including terrestrial-derived resources.\ud Although field studies indicate that fish in estuarine habitats grow well and have\ud access to quality prey resources, experimental manipulations of salinities were used to quantify the physiological costs of residing in the freshwater-saltwater transitional zone.\ud In the laboratory, I designed an experiment to investigate the physiological responses to\ud fluctuating salinities. Experimental treatments consisted of freshwater (FW), saltwater\ud (SW) (22-25%o); and a fluctuating salinity (SW/FW) (2 - 25%o). These treatments were\ud based on typical salinity fluctuations found in estuarine habitats. I measured length,\ud weight, plasma electrolytes and cortisol concentrations for indications of growth and\ud osmoregulatory function. The fluctuating salinity treatment had a negative effect on\ud growth rate and initial osmoregulatory ability when compared with constant freshwater\ud and saltwater treatments. The results indicated that fluctuating salinities had a small but\ud marginally significant reduction in growth rate, possibly due to the additional energetic\ud requirements of switching between hyper- and hypo-osmoregulation. However, 24-hour\ud saltwater challenge results indicated that all fish were capable of osmoregulating in full-strength\ud seawater.\ud In a second experiment, I manipulated feed consumption rates of juvenile spring\ud Chinook salmon to investigate the effects of variable growth rates on osmoregulatory\ud ability and to test the validity of RNA:DNA ratios as indication of recent growth. The\ud treatments consisted of three different feeding rates: three tanks of fish fed 0.7 5% (LOW)\ud body weight; three tanks fed 3% (HIGH) body weight; and three tanks were fasted\ud (NONE) during the experiment. These laboratory results showed a significant difference\ud in the osmoregulatory ability of the NONE treatment compared to the LOW and HIGH\ud treatments which indicates that a reduction in caloric intake significantly effected\ud osmoregulatory capabilities during a 24 hour saltwater challenge. Furthermore, this\ud suggests that there is a minimum energetic requirement in order to maintain proper ion- and\ud osmoregulation in marine conditions.\ud Estuarine marshes have the potential to provide productive feeding grounds with\ud sufficient prey input from terrestrial systems. However, utilization of these marshes in\ud sub-optimal conditions could alter behavior or impair physiological condition of juvenile\ud Chinook salmon prior to their seaward migration by providing insufficient prey resources\ud in a potentially stressful, fluctuating environment. Therefore, the physiological costs associated with estuarine habitat use should be well understood in order to aid future\ud restoration planning