Fine-scale distribution of pelagic fishes relative to a large urban pier

Abstract

Intense shading under large piers is known to negatively affect benthic fishes. However, effects on pelagic fishes are poorly known. We employed the equivalent of acoustic video, dual frequency identification sonar (DIDSON), under a kayak to evaluate the response of pelagic fishes at a large (351 × 255 m) urban pier (Pier 40) in Hudson River. A repeated measures design (322 occupations) sampled 3 transects each across both the northern and southern open water-pier edge-under pier continuum during the day and night from June 2009 to September 2010. Over 22 000 individual fish, ranging from small schooling forage species (e.g. Anchoa mitchilli, Menidia menidia) to large predators (Morone saxatilis, Pomatomus saltatrix) were detected with DIDSON and verified with conventional sampling nets. Small (< 250 mm) schooling pelagic fishes avoided areas under the pier where light is dramatically reduced during both day and night (when municipal lights illuminate the water away from the pier). Less abundant large (> 250 mm) predatory fish responded somewhat differently from small schooling fish in that large predatory fish were slightly more abundant under the pier than in open water, but only from the edge extending to 5 m under the pier where there was still some light. Beyond that, abundance declined sharply. Together, these observations indicate that areas under large piers are suboptimal habitats for many of the abundant pelagic fishes, a pattern similar to that for benthic fishes.

Effects of Seawalls and Piers on Fish Assemblages and Juvenile Salmon Feeding Behavior

“…Our findings of reduced fish abundances under piers are consistent with those of Able et al (2013), who found reduced pelagic fish abundances under a large pier in the Hudson River estuary, suggesting that these effects occur in other systems. Unlike the Hudson River estuary study (Able et al 2013), we did not find higher predator abundances in areas under piers, but predators were rare overall at the depths and habitats that we sampled, and the effects of piers on predators at greater depths are a potential topic for further investigation in our region.…”

“…Thus, for each observation, two types of data were recorded: the total number of fish observed (a number from 1 to 1,000) and the encounter (always counting as one, regardless of shoal size). The encounter metric is useful because shoaling behavior is not well understood in the context of habitat selection, and a large group of fish may not be more indicative of habitat use than a single fish (Able et al 2013 Multivariate analysis was conducted in Vegan (Oaksanen et al 2013). Nonmetric multidimensional scaling was used to visualize differences in assemblage structure (1) between seawall sites and reference beaches and (2) among pier, corner, and open sections within seawall sites separately for high-and low-tide data, excluding species that were observed in less than 5% of the surveys.…”

Effects of shoreline armouring and overwater structures on coastal and estuarine fish: opportunities for habitat improvement

“…Total abundances of fish can be substantially reduced in areas shaded by piers (Southard et al . ; Able, Grothues & Kemp ). Juvenile salmon Oncorhynchus spp.…”

Detecting a nearshore fish parade using the adaptive resolution imaging sonar (ARIS): An automated procedure for data analysis

“…reefs and harbors, though these areas have great ecological significance. For example, natural and man-made structures may offer refugia, e.g., mangrove prop roots (MacDonald et al, 2009), sea-grass beds (Sogard and Olla, 1993) and pile fields (Able et al, 1999), while other altered coastlines (harbors) may prove sub-optimal for fish (Able et al, 2013). Sonar cameras like the DIDSON and ARIS (SoundMetrics Inc.) are valuable tools used to sample fish abundance non-destructively, size and behavior within hard-to-observe, structurally complex, turbid and dark environments.…”