Figure 29.2. Mean abundance (CPUE) of the 10 most abundant fish species under piers, at pier edges, in open water, and in pile fields based on data collected in 1993,1994, and 1996.
cunner were very abundant in 1993 and 1994, but were completely absent from collections in 1996.
There were marked dissimilarities in fish distribution in different habitats. Mean fish abundance (CPUE) was consistently lower under piers (though variability was high) compared to openwa-ter, pile field, or edge habitats. Only one species, the American eel, was ever collected from under-pier areas more frequently than in any other habitat (Fig. 29.2). Atlantic tomcod and naked goby (Gobiosoma bosc) were not uncommon in under-pier traps but they were collected in higher numbers at other sites. In addition, the total number of species collected from under piers was lower (n = 16 species) than the total number collected in pile fields (n = 21 species) or in open water (n = 27 species). Many of the species collected from under-pier habitats were only collected once and were never observed under piers again during the three year sampling survey. In contrast, many of the species found in the other habitat types were observed there on more than just one occasion and some, such as spotted hake, tautog (Tautogaonitis), and northern pipefish (Syngnathus fuscus), were collected repeatedly (Able et al., 1998). It should be noted that the traps used in this study were designed specifically to sample small, young-of-the-year, benthic fishes. As such, other fish species that are common to the Hudson River may not have been effectively sampled, especially pelagic fishes that occur higher in the water column. Still, our data suggest that fish abundance is consistently depressed under piers over multiple years, indicating that piers are lower-quality habitats for fishes relative to edges, pile fields, or open water (Able et al., 1998; Duffy-Anderson and Able, 1999; Able, Manderson, and Studholme, 1999;Duffy-Anderson etal., 2003).
Interestingly, the three fish species collected under piers (American eels, Atlantic tomcod, and naked goby) and the decapod species (Able and Duffy-Anderson, 2005) collected share a common characteristic; they do not rely strictly on the use of vision to forage; rather, they demonstrate various abilities to utilize alternative sensory systems to locate and capture prey. For example, American eels and Atlantic tomcod can detect chemicals in solution (Herrick, 1904; Teichmann, 1954; Silver, 1979). Some gobies have been shown to have a similar ability to detect chemicals (Utne and Bacchi, 1997), though it has also been demonstrated that their reactive distance to predators declines with decreasing light intensity (Aksnes and Utne, 1997), suggesting that vision is still an important component of the overall sensory behavior of the animals. Intense shading from the solid decks of the piers examined in our study drastically reduced light penetration to the waters below. Light levels under piers were approximately 4-5 orders of magnitude lower than outside of piers (0.0010.02 |E-2 s-1 under piers versus 20-60 |E-1 s-1 outside of piers, Duffy-Anderson and Able, 1999).
Occasionally the levels were so low that they were below the detection of our light meters. Light is a limiting factor that affects the ability of visually-foraging fish to search for prey (Boeuf and Le Bail, 1999). At low light intensities, important factors associated with prey recognition, such as prey contrast and hue, are reduced (Gerking, 1994) limiting the ability of fish to identify prey items. Similarly, reactive distance, the maximum distance at which visual predators can detect their prey (Vinyard and O'Brien, 1976) declines with declining light intensities (O'Brien, 1979), reducing the search volume of visually feeding fish. Thus, visually foraging fishes may not occur under piers because conditions of intense shading interfere with one or more of the steps in the predation cycle. We therefore speculate that under-pier areas may only serve as functional habitat for a few select species, perhaps only those with supplementary sensory systems that allow them to forage more effectively in darkness, while simultaneously being inhospitable to a variety of other estuarine species.
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