Modification of nektonic fish distribution by piers and pile fields in an urban estuary

• Nektonic fish avoided deeply shaded habitat under large piers in an urbanized estuary.
• Nektonic fish also avoided unshaded piling fields from former piers.
• Among-habitat differences were weaker at night.
• Findings are similar to those for benthic fish under piers, but not for those among uncovered pilings.
• Schooling by fish appeared to mitigate the distributional response among habitats.

Large urban piers degrade habitat value for several estuarine benthic fish species by shading, but their effects on mobile nektonic species is less well understood due to sampling challenges. Dual Frequency Identification Sonar (DIDSON) allowed equal access to sampling in the water column of structured shaded and unshaded vs. open environments in both dark and light conditions by methods similar to video but without light. Sampling (n = 228, 5-min transects) occurred under and around four large municipal piers of varying dimensions in the Hudson River estuary during day and night from summer and fall in 2007–2009. The distribution of small (5–25 cm in length) and large (25–850 cm) fishes were analyzed separately in recognition of functional guild differences. Small fishes occupied open water, shaded under-pier, and un-decked relict piling habitats, but were significantly more abundant during the day in open unshaded water than under adjacent piers or in piling habitats. Small fish occurred under 3 of 4 piers of varying size and configuration at 10–20% of the median abundances of adjacent open water. However, while schools were rare under piers they could be very large, so that abundance greatly exceeded mean open water abundance variance so as to preclude confidence in differences among piers. The differences among habitats were not significant at night, and the difference among piers was also not significant at night. School membership for small fish appeared to mitigate adverse effects of shading and may influence scaling of their response to shading and could therefore influence pier design. Large (> 25 cm) predatory fish were uncommon but responded similarly to habitat effects as did small fish. Habitats did not segregate fish by guild as small forage fish co-occurred in 65.8% of samples with large piscivores. Studies that provide species-specific and mechanistic interpretation of dynamic habitat use as well as further quantification of scaling effects could improve our understanding of how fishes respond to piers and other structures on urban shorelines.