Benthic Structure and Pelagic Food Sources Determine Post-settlement Snapper (Chrysophrys auratus) Abundance

Nursery habitats provide increased survival and growth and are a crucial early life-stage component for many fish and invertebrate populations. The biogenic structures that provide this nursery function, however, are increasingly degraded. Therefore, any effort to conserve, restore or replace habitat with artificial structure should be guided by an understanding of the value provided by that nursery habitat. Here, we experimentally manipulated structure across a number of sites by inserting pinnind bivalve mimics into the seabed and deploying video cameras to observe the response of post-settlement stage snapper, Chrysophrys auratus (Forster in Bloch and Schneider 1801). We also collected a range of environmental variables across these sites to determine the relative importance to snapper of benthic vs. pelagic productivity. While the abundance of snapper was low, our results demonstrated a strong association to structure relative to control plots. The environmental variable with the highest correlation to snapper abundance was the abundance of zooplankton eaten by snapper. This result was well supported by the dominance of zooplankton over small benthic invertebrates in snapper gut contents, and the weak influence of benthic infauna in our regression models. These regressions also demonstrated that when combined with zooplankton abundance, turbidity had a negative relationship to snapper abundance. This highlights the importance of relatively clear water in estuaries, which allows post-settlement snapper to more efficiently consume the zooplankton that are present in the water column. The third component that post-settlement snapper require is of course the presence of benthic structure. While benthic habitat structure was the strongest factor affecting juvenile snapper abundance, we did not find any correlations to suggest that this importance was related to energetic sheltering and access to locations with high food flux

Do nursery habitats provide shelter from flow for juvenile fish?

<div><p>Juvenile fish nurseries are an essential life stage requirement for the maintenance of many fish populations. With many inshore habitats globally in decline, optimising habitat management by increasing our understanding of the relationship between juvenile fish and nursery habitats may be a prudent approach. Previous research on post–settlement snapper (<i>Chrysophrys auratus</i>) has suggested that structure may provide a water flow refuge, allowing snapper to access high water flow sites that will also have a high flux of their pelagic prey. We investigated this hypothesis by describing how Artificial Seagrass Units (ASUs) modified water flow while also using a multi–camera set up to quantify snapper position in relation to this water flow environment. Horizontal water flow was reduced on the down–current side of ASUs, but only at the height of the seagrass canopy. While the highest abundance of snapper did occur down–current of the ASUs, many snapper also occupied other locations or were too high in the water column to receive any refuge from water flow. The proportion of snapper within the water column was potentially driven by strategy to access zooplankton prey, being higher on the up–current side of ASUs and on flood tides. It is possible that post–settlement snapper alternate position to provide opportunities for both feeding and flow refuging. An alternative explanation relates to an observed interaction between post–settlement snapper and a predator, which demonstrated that snapper can utilise habitat structure when threatened. The nature of this relationship, and its overall importance in determining the value of nursery habitats to post–settlement snapper remains an elusive next step.</p></div

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Vertical distributions of mean water velocity around an ASU.

<p> is longitudinal velocity, is transverse velocity, and is vertical velocity, all measured at locations A, B and C. Profile number refers to the time period when each set of water velocity measurements were collected (AMP1 (profiles 1–7) and AMP2 (profiles 8–14), but see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0186889#pone.0186889.g008" target="_blank">Fig 8</a> for a graphical representation). Measurements were made with ADVs during a two day deployment in June 2016.</p

Do nursery habitats provide shelter from flow for juvenile fish? - Fig 10

<p><b>Vertical distributions of turbulence energy dissipation (ε) measured at locations A, B and C.</b> Profile number refers to the time period when each set of turbulence measurements were collected (AMP1 (profiles 1–7) and AMP2 (profiles 8–14), but see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0186889#pone.0186889.g008" target="_blank">Fig 8</a> for a graphical representation). Measurements were made with ADVs during a two day deployment in June 2016.</p

Aerial view layout of video cameras positions used to film post–settlement snapper associated with an ASU.

<p>Camera position naming indicated for just one water flow direction. Camera naming reverses on the opposing tidal direction. No camera deployments encompassed a change in tidal direction.</p

Proportion of 0+ snapper within the water column by water velocity.

<p>Proportion data are presented as open circles and were categorised for maximum counts from one minute video observations. The left column of plots are for flood tide camera deployments, the right column for ebb tide camera deployments. Each row of plots represents the proportion of snapper in the water column as observed from a different camera position. Data presented have been subsampled to reduce temporal autocorrelation. The red line for each panel represents a 2^nd order polynomial quantile regression spline fitted through the 50^th percentile of the data.</p

Map of Whangarei Harbour.

<p>Intertidal sand banks indicated by light grey shading and the location of the Artificial Seagrass Unit array denoted by a black star. North Island, New Zealand and Whangarei Harbour inset.</p

Abundance of 0+ snapper by water velocity.

<p>Snapper abundance is the maximum count observed within a one minute video sample. The left column of plots is for flood tide camera deployments, the right column for ebb tide camera deployments. Each row of plots represents snapper abundance as observed from a different camera position. Data presented have been subsampled to reduce temporal autocorrelation. The red line for each panel represents a 2^nd order polynomial quantile regression spline fitted through the 50^th percentile of the data.</p

ADCP measurements of the background conditions experienced during the ADV deployments.

<p>Panels: (A) water depth, (B) current speed, (C) current direction (in oceanographic convention “flowing to”) and (D) significant wave height (<i>H</i>_<i>s</i>). The timing of when the water flow profiles were collected is indicated on panel (A).</p