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o (.-j f~ -.1 o <.0 Platania, Dudley, and Maruca. 2000. Drift of Fishes in the San Juan River ]991.IQ97. FINAL et at., 1988; Tyus and Haines, 1991; Bestgen et at., 1998). Severe weather conditions (heavy rainstorms and lightning) occasionally precluded the ability to obtain daily samples. Drift-nets were set for two hours intervals at dawn and occasionally at dusk. Drift-nets often remained in the river for periods greater than two hours during periods of very low flow and low suspended sediment, while the opposite was trUe when there were large volumes of suspended debris in the river. At the end of eac.h set, the contents of each drift-net were rinsed, labeled, and preserved in 10% formalin. Drift material was allowed to cure for at least two days before samples were sorted and fishes separated from the debris. Cleaned samples were returned to the laboratory at the Museum of Southwestern Biology (MSB) for identificatioll and curation. All fish specimens were identified to species (where possible) and counted. Specimens were assigned to categories of "drift" or "incidental" based primarily on their developmental stage. "Drift" referred to individuals with minimal or no control over their longitudinal movement, while "incidental" referred to individuals whose developmental stage indicated that they were not actually a component of the drift but were incidentally captured. Their capture may have been due to strong currents (rainstorm events) or other factors. The primary criterion for identifying a fish as an incidental capture was the presence of fully differentiated fins. The approximate maximum standard length (SL) was determined for each of the San Juan River species with drifting larvae (Table 2). The principal purpose in making the distinction between drift and incidental captures was because this study was designed to assess drifting larval fish. Fish that accidentally fonnd thcir way into the drift-net or that were transported downstream due to high flow events provided little information to the overall objectives of the passive drift-netting study. Fish classified as incidental were noted in totals but not included in subsequent analyses. The term YOllng-of-year (YOY) can include bl)th larval and juvenile fIsh. It refers tl) any fish, regardless of developmental stage, between hatching or parOlrition and the date (I January) that they reach age I (i.e., YOY = Age 0 fish). Larval fish is a specific developmental (morphogenetic) period between the time of hatching and when larval fish transform to jnvenile fish. We have chosen to follow larval fish terminology as defmed by Snyder (1981). There are three distinct seqnential larval developmental stages; protolarvae, meso larvae, and metalarvae. Fish in any of these developmental stages are referred to as larvae or larval fish. Juvenile fish are those that have progressed beyond the metalarval stage and no longer retain traits characteristic of larval fishes. Jnveniles were classified as individuals that l) had completely absorbed their fin folds, 2) had developed the full adult complement of rays and spines, and 3) had developed segmentation in at least a fcw of the rays. In this work, YOY -juvenile fish refer to Age-O non-larval fish. Morphogenesis nomenclature (larval and juvenile) should not be confused with terms related to downstream passive movement (=drift). For these investigations, the word "drift" is defined as the passive movement of fish in a downstream direction. While drifting fish may have some ability to control their vertical movcment, they have not achieved sufficient development to allow them to actively move out of the c.urrent and into low-velocity habitats. Fish that have progressed far enough through morphogenesis to be able to control both their vertical and horizontal movement are usually in one of the latter larval stages and are no longer considered driti (non-drift = incidental). All fish classified as drift are larvae, but not all larval fish are drift. Calculation of Catch-Per-Ullit-Effo/'1 (CPUE) Calculation of catch rate required all appropriate measure of effort which, for drift-netting, was volume of water sampled. Catch-per-unit-effort (CPUE) was calculated as number of drifting fish caught per unit volume of water sampled. This measure was deemed the most appropriate estimate for density of larval fish and assessment of reproductive success for each population of fish studied. Volume of water sampled was measured using a mechanical flow-meter attached to each net 9