Diets

Why are diets important? What information do they tell us? What questions do they answer? Fish’s view –Source of energy (bioen) Managers view –Stocking Aquaculture –Nutrients As an indicator of environmental change –Nutrient loading, change in pop density

How do we get diets? Fish collection: –what should we be cautious of? Sources of error/ bias due to methods selection? Regurgitation Post capture digestion Alteration in behavior due to capture –Traps Did you use bait?

Sampling strategies Diel effects Seasonal effects –Both of the above affect predator and prey behavior Fish size –Ontogenetic shifts? What age/size class is your diet representing? Things to consider/keep in mind

Sampling strategies cont… Digestion rates –Too fast - protozoans Under representation in diets Could be determined by watching behavior in a tank –Too slow Over representation in diets Could be solved by determining gut passage times Do stomach contents accurately depict the fish’s diet?

Removal of gut contents Puking Dissection

Identification of Diet Components Crushed/digested organism –Find characteristic structure for each organism Level of identification depends on the research question –Higher resolution=greater time investment

Quantitative diet description Frequency of Occurrence Percent composition by number Percent composition by weight

Frequency occurrence What proportion of the diets contained one or more of a given food type Describes presence absence Example: 18/22 bluegill contain chironomid, frequency of occurrence =.82 or 82% Selective or opportunistic feeding behavior is reflected in lower frequency occurrence

Frequency of occurrence Drawbacks High frequency of occurrence may not mean this diet item is of nutritional importance, only that it is consumed with some regularity Example: benthic fish and algae Describes the uniformity with which fish feed, not importance of diet items

Percent Composition by number Number of food items in a group relative to total number of diet items consumed (%) Potential for fragmentation, count a characteristic part of prey item –Dragonflies have four wings, so 4 wings = 1 dragonfly Percent composition by number can be used together with estimates of feeding rates to assess the effects of predators on prey population dynamics

Percent Composition by Weight Weight of each type as a percentage of the total weight of the diet –Wet and dry weights can be used Dry are more precise and offer more information about nutritional value Remember some component of the diets have already been digested!!!! This is the only method (described here) that begins to identify food importance in fish nutrition

Further Analysis and Interpretation Selectivity indices: Do fish feed at random or preferentially? –Relative abundance of prey type in diet to the relative abundance of prey type in the environment Requires data on relative prey abundance in the environment Diet overlap indices –When comparing different species in a community may want to know how much two species overlap Extent to which species use the same food source

Laboratory Groups of two/three “work up” 1-2 diets –Fill out data sheet including weight of each diet item Combine data with other groups Analyze the diets –Freq occ, % comp by number, % comp by weight