Fish Assemblages on Estuarine Artificial Reefs: Natural Rocky-Reef Mimics or Discrete Assemblages?

<div><p>If the primary goal of artificial reef construction is the creation of additional reef habitat that is comparable to adjacent natural rocky-reef, then performance should be evaluated using simultaneous comparisons with adjacent natural habitats. Using baited remote underwater video (BRUV) fish assemblages on purpose-built estuarine artificial reefs and adjacent natural rocky-reef and sand-flat were assessed 18 months post-deployment in three south-east Australian estuaries. Fish abundance, species richness and diversity were found to be greater on the artificial reefs than on either naturally occurring reef or sand-flat in all estuaries. Comparisons within each estuary identified significant differences in the species composition between the artificial and natural rocky-reefs. The artificial reef assemblage was dominated by sparid species including <i>Acanthopagrus australis</i> and <i>Rhabdosargus sarba</i>. The preference for a range of habitats by theses sparid species is evident by their detection on sand-flat, natural rocky reef and artificial reef habitats. The fish assemblage identified on the artificial reefs remained distinct from the adjacent rocky-reef, comprising a range of species drawn from naturally occurring rocky-reef and sand-flat. In addition, some mid-water schooling species including <i>Trachurus novaezelandiae</i> and <i>Pseudocaranx georgianus</i> were only identified on the artificial reef community; presumably as result of the reef's isolated location in open-water. We concluded that estuarine artificial reef assemblages are likely to differ significantly from adjacent rocky-reef, potentially as a result of physical factors such as reef isolation, coupled with species specific behavioural traits such as the ability of some species to traverse large sand flats in order to locate reef structure, and feeding preferences. Artificial reefs should not be viewed as direct surrogates for natural reef. The assemblages are likely to remain distinct from naturally occurring habitat comprised of species that reside on a range of adjacent natural habitats.</p></div

Mean (± S.E.) Cath per unit effort (number of fish/Megalitre) across different mesh and velocity treatments.

<p>Mean (± S.E.) Cath per unit effort (number of fish/Megalitre) across different mesh and velocity treatments.</p

Predicted relationship of probability of screen contact with length and rheotaxis of fish (see methods for description of rheotactic behaviour).

<p>Solid lines are at 0.1 m.sec⁻¹ and broken lines are at 0.5 m.sec^−1.</p

Number of fish entrained within the experimental pump system.

<p>Catches are pooled within each velocity and mesh combination. The electrofishing/seine column demonstrates the composition and relative abundance of fish captured at all the experimental site using electrofishing and seine netting.</p

Tabular representation of the two Latin square experimental designs showing the order of allocation of mesh treatments within replicate runs for each of two approach velocities.

<p>Tabular representation of the two Latin square experimental designs showing the order of allocation of mesh treatments within replicate runs for each of two approach velocities.</p

Length frequency plot showing the size range of fish observed by sonar to make a) contact or b) avoid contact with the experimental screen.

<p>Length frequency plot showing the size range of fish observed by sonar to make a) contact or b) avoid contact with the experimental screen.</p

Length frequency plot for fish sampled a) by electrofishing and seine netting at pumping sites, and b) those collected after being entrained by the pump (all treatments pooled).

<p>Length frequency plot for fish sampled a) by electrofishing and seine netting at pumping sites, and b) those collected after being entrained by the pump (all treatments pooled).</p

Screenshot showing acoustic echogram obtained from sonar (a) alongside post-processing modules created in Echoview, including a background reduction and target identification module (b) and fish tracking module (c).

<p>Screen panels can be seen as light coloured bands at the image bottom.</p

Schematic representation of the experimental pumping station showing major components.

<p>The experimental screen was cylindrical and comprised a series of removable panels to adjust approach velocity and mesh size. Fish that were entrained into the system travelled through the intake pipe, through the pump, along the lay-flat discharge pipe and were collected in fyke nets fitted to a settling tank. Fish behaviour in front of the screen face was quantified using DIDSON.</p

Odds Ratios of rheotactic categories compared to random orientation.

*<p>The odds ratio is the increase in the probability of contact when compared to random orientation. For example, fish showing positive rheotaxis at the 0.1 m/s velocity are 14×less likely to make contact (1÷0.07 = 14).</p>†<p>Significant (sig) or non-significant (ns) at the p = 0.05 level.</p