I investigated the effect of turbidity on the foraging behaviour of juvenile Chinook salmon (Oncorhynchus tshawytscha) in the laboratory. Specifically, I examined a behavioural "tradeoff" between visual ability and "perceived" risk. I assessed visual ability by measuring the reaction distance of juvenile Chinook to planktonic Artemia prey. I found Chinook exhibited a log-linear decline in reaction distance with increasing turbidity. To determine the effects of turbidity and microhabitat on foraging rate, I conducted separate experiments for surface (Drosophila), planktonic (Artemia), and benthic (Tubifex) prey across a range of turbidity levels (0, 25, 50, 100, 200, 400, 800 mg•L⁻¹). Foraging rates were reduced at higher turbidity conditions for all three prey. However, for surface and benthic prey, foraging rates were also low in clear water; highest rates were attained at intermediate turbidity levels (50-200 mg•L⁻¹). The degree to which intermediate turbidities were associated with higher foraging rates was size-dependent. Smaller individuals (150-57 mm FL) exhibited relatively higher foraging rates in clear conditions than did larger individuals. However, planktonic foraging rates by juveniles were consistently high in clear water, regardless of fish size. In experiments manipulating light level independent of turbidity, I allowed salmon to forage under conditions which were either turbid, or clear but with light intensity correspondingly reduced. Foraging rates were similar between the two treatments for planktonic prey, but differed for benthic and surface prey. Generally, foraging rates exhibited by juvenile Chinook salmon could not be explained on the basis of visual ability alone. I suggest that young salmon also exhibited foraging behaviour consistent with their perception of risk to predation. In arena experiments, juvenile Chinook distributed themselves randomly in turbid conditions; in clear conditions they associated with the bottom. When bird and fish predator models were introduced the fish altered their spatial distribution, occupying deeper regions regardless of turbidity. However, their response in turbid conditions was less marked and lasted for a shorter time. Turbidity apparently mitigated the perceived risk of predation in juvenile Chinook. I developed a conceptual tradeoff model that predicted the general effect of turbidity on foraging behaviour. Assuming differences in either prey quality or perceived risk of predation in three microhabitats (surface, water column, bottom), the model resolved the apparent dissimilarities between planktonic and other foraging behaviours. Perceived risk of Chinook to predation was significantly different between surface and water column microhabitats. When exposed to a non-visual "fixed-risk" stimulus (sound), salmon apparently perceived less risk as turbidity increased. I conclude that in turbid waters juvenile salmon exhibit foraging behaviour in a manner consistent with a tradeoff between their visual ability and perceived risk. / Science, Faculty of / Zoology, Department of / Graduate

Feed accounts for 40% to 60% of the operating costs of Pacific salmon farms. Presently, commercial diets for Pacific salmon in seawater are formulated according to the nutrient requirements of juvenile chinook salmon and digestibility information derived from studies on rainbow trout in fresh water. Information on feedstuff digestibility by Pacific salmon is completely lacking, regardless of life history stage. Hence, this thesis was undertaken to determine the apparent digestibility of organic matter, crude protein and energy in conventional and novel feedstuffs using post-juvenile chinook salmon (Oncorhynchus tshawytscha) in seawater. The validity of determining apparent nutrient digestibility using the "Guelph system" of fecal collection and chromic oxide (Cr₂O₃) as the indigestible indicator in the diet was assessed in experiment I. In this regard, fecal samples were collected either from a settling column affixed to each novelly designed digestibility tank at 6 and 18 hour intervals or directly from the terminal section of the intestinal tract by stripping or intestinal dissection. Digestibility coefficients were noted to be significantly increased (P<0.001) when the feces remained in the water for 18 hours instead of 6 hours, owing to nutrient leaching. The collection of feces directly from the fish resulted in lower digestibility coefficients than those found when feces were obtained from the "Guelph system". This was attributed to a flaw in the design of the digestibility tank drain system, whereby the feces did not settle quickly into the collection column. The problem was rectified before conducting the subsequent experiments described below. In three additional experiments on chinook salmon in seawater, the overall goal was to ascertain the organic matter, crude protein and energy digestibility coefficients and the digestible energy values for commercial sources of fish meal (herring meal, anchovy meal, menhaden meal, Norwegian low temperature fish meal), poultry by-product meal (two suppliers), feather meal, blood meal, dried whey, canola meal, soybean meal, soybean protein isolate, extruded wheat and wheat middlings. Also, three novel sources of rapeseed protein products (two types of glucosinolate-free canola meal and rapeseed protein concentrate) were evaluated in this regard. The initial minimum size of the chinook in these studies varied between 10.3 g and 40.5 g. The digestibility tank complex consisted of 27 open-circuit 150 L fiberglass digestibility tanks, each supplied with aerated, 8.0°C to 12.5°C filtered seawater at a rate of 6 L/min. The fish were fed by hand twice daily to satiation either a reference diet or a test diet (70% reference : 30% test ingredient). In some instances involving plant protein products, the test ingredient was included at two dietary levels (15% and 30%). Within each test, each diet treatment was assigned to three groups of fish using a completely random or randomized complete block design. Chromic oxide (0.5%) was included in all diets as the indigestible marker. At the end of each experiment, fish were sacrificed and their feces were removed either by stripping or intestinal dissection for comparisons of methodology. In general, the fish meals had higher available energy content for chinook salmon than the other animal and plant protein sources assessed. The importance of screening feedstuffs for potential nutritive value by digestibility measurements was particularly evident from an examination of the digestibility coefficients obtained for menhaden meal versus the other fish meal sources, the two sources of poultry by-product meal, wheat products, canola products and blood meal in this study. The assessment of soybean products, regardless of dietary inclusion level (15% or 30%), could not be ascertained in chinook salmon because of poor diet acceptance. Canola protein sources appear highly promising as partial or complete (in the case of rapeseed protein concentrate) replacements of fish meal, based on digestibility assessment. The use of the "Guelph system" for fecal collection resulted in organic matter digestibility coefficients similar to those obtained by intestinal dissection. Thus, nutrient leaching must have been minimal with the "Guelph system" and it is concluded that this is a satisfactory procedure for digestibility assessment using chinook salmon in seawater. / Land and Food Systems, Faculty of / Graduate