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FIGURE 5.4. A. Brier Creek, a tributary to the Red River arm of Lake Texoma in Oklahoma. Numbers show sampling stations. Map based on C. L. Smith and Powell (1971) and Ross et al. (1985).
B. Recolonization of a dewatered section of Brier Creek, Oklahoma, following resumption of stream flow. Drying occurred in a 1.5 km reach at station two. Severe drought dewatered the stream again by September. Data from Matthews (1987).
Movement of freshwater fishes is generally studied by marking individuals in some way and then attempting to recapture them later (termed mark-recapture). As pointed out by Funk, sampling is almost always greater near the point of release compared to sampling at great distances from the release, especially if there are many potential routes of fish movement (as in lakes or in streams with numerous tributaries). As a consequence, sampling effort is unequal over distance from the point of release, so that short movements tend to be recorded more often than long movements. The change in sampling intensity with increasing distance from the point of release is referred to as “distance weighting” (Porter and Dooley 1993; Albanese et al. 2003). Thus unless the study compares recoveries against the probability of recapture (i.e., capture probability decreases as fishes disperse outward from the point of release) or, preferably, is designed to alleviate the issue of distance weighting, there would be strong bias for interpreting recovery data as supporting limited fish movement (Box 5.2).
BOX 5.2 • Distance Weighting in Studies of Fish Movement
An appropriate experimental design is critical for assessing fish movement using mark-recapture approaches. This is especially so because the likelihood of capturing a marked fish declines with distance from the point of release, leading to the risk of underestimating longer movements. A robust experimental design is an important issue even with essentially linear stream systems—additional complexity of the aquatic system (e.g., tributary streams, lakes with numerous coves, etc.) further increases the challenge of obtaining reliable data. Following Rodríguez (2002), in quantitative terms, the density of marked fish multiplied by meters away from the region of their release, n(x), is given by
where No is the number of fish originally marked, s is the probability of their surviving to the sampling period, π is the catchability of the fish, and f(x) describes the decline in density as a function of the distance from the release area (referred to as a dispersal function). The key point that this equation makes is that the number of recaptures at any given location must be evaluated relative to the probability of recapture.
Albanese et al. (2003) evaluated the impact of distance weighting on the assessment of movement by three species of southeastern stream fishes. He used a modeling study to illustrate the impact that increasing the number of 50 m sampling sections would have on distance weighting. Fish were considered marked in ten 50 m sections. The modeling approach showed how the zone of uniform sampling (i.e., sampling at or near 100%) changed as the sampling area was increased. If sampling only occurred within the 500 m marking section, then the impact of distance weighting was extreme. The proportion of total possible movements sampled (PSd) was only 100% for sampling within ± 50 m (i.e., the zone of uniform sampling) and declined sharply for movement distances > ± 50 m. Clearly, a study design that only included sampling within the same stream reach used to mark fish would be strongly biased toward short-term movements. With a 1,000 m sampling effort, the PSd values were 100% for fish movements within ± 250 m, and with a 2,000 m sampling effort, PSd values are 100% for fish movements within ± 750 m. Both of these are much more robust designs in terms of understanding movement. However, even with the 2,000 m sampling effort, any fish movements greater than ± 1,200 m would have been undetected.
In the following figure, based on Albanese et al. (2003), the modeling study shows the proportion of total possible movements (PSd) that would be detected for three different sampling designs: one in which the sampling reaches were the same as the marking reaches (500 m), one in which the sampling reaches (1,000 m) were twice that of the 500 m marking reach, and one in which the sampling reaches (2,000 m) were four times that of the 500 m marking reach. Solid lines show PSd values; dashed lines show the sampling lengths. Movement can be either upstream (+) or downstream (−) from the marking section.
The effect of the size of the resampling areas (indicated by dashed lines) on the ability to detect marked fishes, relative to the distances that the fishes move. As determined by a modeling study, the largest resampling area (2,000 m) can detect 100% of fish movements up to 750 m upstream or downstream from the marking site (indicated by shading). The smallest sampling area (500 m and the same as the marking area) could only detect 100% of fishes that moved less than approximately 50 m. Based on Albanese et al. (2003).
In theory, the probabilities of capture from distance weighting could be used to adjust the observed captures of fish (see Albanese et al. 2003). In actuality, outside of the zone of uniform sampling, the numbers of fish captured were so low that adjustments generally were not possible—adjustments are not possible if no fish are captured! The take-home message is that an understanding of distance weighting using PSd values is most useful for the a priori design of the sampling study. The a posteriori application of correction factors generally cannot correct for a poor study design.
In spite of the cautionary words by Gerking (1959) about the problem of experimental design and the presence of fish straying, and the suggestion by Funk (1955) about sedentary and mobile groups, the restricted movement paradigm became entrenched in the literature. One of the first to take issue with the restricted movement paradigm, specifically in regard to salmonids, was Gowan et al. (1994), who pointed out that although most tagging studies captured the majority of fish near the point of release, in 78% of the salmonid studies that they reviewed, over half of the fishes were never seen again after being marked. Whether these fish represented mortalities or fish that simply moved much greater distances is the crux to understanding the level of movement of fishes. Gowan et al. (1994) also suggested that the mobile fraction of fish populations had been downplayed through the use of such deprecating terms as “strays” and argued that more attention needed to be given to the experimental design of fish movement studies and to the underlying mechanisms involved in fish movement. The greater realization of the often high degrees of movement shown by freshwater fishes has had important consequences for the better understanding and management of fish populations (Fausch et al. 2002).
A study of fish movement in a small Ouachita Highlands stream (Arkansas) involved four species of stream fishes (Creek Chub, Semotilus atromaculatus; Blackspotted Topminnow, Fundulus olivaceus; Green Sunfish, Lepomis cyanellus; and Longear Sunfish, L. megalotis) (Smithson and Johnston 1999). The study area was 500 m long and consisted of 10 pools, and the possibility of movement of fishes outside of the study area was determined to be unlikely. Most fishes were recaptured in the same pool where they were initially marked; however, there were differences among the species. Compared to the other three species, Blackspotted Topminnow moved significantly greater distances. For all species, there were some individuals that moved greater distances than others, although there were no apparent morphological correlates associated with greater movement and, in fact, the same individuals switched between static and mobile behaviors over the course of the study. This suggests that individual fish periodically engage in exploratory travel, perhaps assessing habitat quality in areas outside of their home pool.
Field and theoretical approaches were used by Skalski and Gilliam (2000) to examine characteristics of movement of primarily four species (Bluehead Chub, Nocomis leptocephalus; Creek Chub; Redbreast Sunfish, Lepomis auritus; and Rosyside Dace, Clinostomus funduloides) in a small southeastern stream. In contrast to Smithson and Johnston (1999), species showed only weak differences in their degree of movement. However, within a species there was evidence for both “movers”
