Climate change has impacted all ecosystems on Earth, despite an average warming of only ~1°C so far (Pecl et al., 2017; Scheffers et al., 2016). One of the most widely documented impacts of warming is the global redistribution of species (Parmesan, 2006; Poloczanska et al., 2013). To stay within their preferred thermal ranges, many species are moving towards the poles, to greater altitudes on land, and into deeper waters in the ocean (Chen, Hill, Ohlemüller, Roy, &

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Many species of fish move between ocean and estuarine habitats; however, there is little evidence of the magnitude of fish undertaking these movements particularly over short time scales. Such information is critical in understanding the connectivity between these major habitats. We used an acoustic camera to observe the entire entrance of a small estuary over a 4-month period during 3 h of ebb and flood tides and during day and night, which allowed us to count all fish passing through the entrance and observe their schooling behaviour. Nearly 30,000 fish transited in 60 h of observation over the study period, with a mean of 1396 (±240 S.E.) fish per 3 h deployment and a maximum of 4636 fish per 3 h. Of these, 20,170 entered the estuary while 7751 exited. Movements of fish were closely related to tidal flow when fish movement peaked during the middle of the tide. While the majority of fish swam with the tide, approximately 32 % swam against the tidal current. The schooling behaviour of transiting fish varied between fish entering and exiting the estuary, with incoming fish far more likely to school than those exiting. This may be an antipredator behaviour but also related to group navigation. This study has provided insights into the tight coupling of estuaries and ocean, and the tidal influence on the mass movements of fish.

The fine-scale distribution of pelagic forage fish is shaped by competing factors as fish optimise foraging while avoiding predation. We investigated the distribution of forage fish in surface waters of a dynamic coastal environment during two spring seasons to examine their distribution in relation to environmental variables. Using a multifrequency echosounder and a towed Laser Optical Plankton Counter (LOPC), we investigated the effects of bathymetry, temperature, chlorophyll a concentration and zooplankton biomass on forage fish density. Relationships between fish density and these variables were consistent between surveys, despite large differences in total acoustic energy attributed to fish. Fish density showed a strong positive relationship with bathymetry and water temperature, and no relationship with surface zooplankton biomass density or chlorophyll a. This mismatch between fish and zooplankton may be caused by differences in the way fish perceive the distribution of prey versus temperature. Despite large concentrations of zooplankton in surface waters near the coast, the topographic constraint of shallow water on fish vertical distribution may increase the risk of predation in this region. Seeking out warmer temperatures along the shelf break may also improve fish physiological performance when cooler spring temperatures are below their thermal optimum. Understanding the distribution of coastal forage fish may contribute to interpreting nearshore movements of their predators.

Plankton is an important component of the food web in coastal reef ecosystems. Ocean currents subsidise local production by supplying plankton to resident and reef-associated planktivorous fishes. Measuring the fine-scale distribution of these schooling fishes provides insight into their habitat use and how they balance risk and reward while foraging for plankton. Maintaining their proximity to benthic structure can provide refuge from predation but may also limit foraging opportunities. We used a portable multibeam echosounder to survey schooling fish at 5 natural and 3 artificial reefs, during day and night and under different current conditions. We isolated midwater acoustic targets and used generalised linear models to explain the distribution of schools as a function of current exposure, distance from structure and seafloor complexity. We also isolated individual schools and used generalised least squares to model how school characteristics differed between night and day, using spatial metrics of school area, perimeter length and height above the seafloor. Modelling revealed that the occurrence of schools was almost twice as likely upstream versus downstream of artificial reefs, although distance to reef structure was the main influence. School occurrence was also more likely on artificial versus natural reefs. Schools at artificial reefs exhibited greater volume and areal coverage at night, and during the day they rose higher in the water column while aggregating more closely around the reef. These findings suggest that artificial and natural reefs featuring enhanced vertical relief and direct exposure to the prevailing current are preferred habitats for planktivorous fish.

Multispecies schools of small planktivorous fishes are important constituents of reefs and coastal infrastructure; however, determining the extent and distribution of these schools is challenging. Here, we describe a novel use of a low-cost portable multibeam echosounder from a small vessel, which can concurrently measure detailed bathymetry and the distribution of mid-water targets with high spatial accuracy, regardless of light availability or water clarity. Fish abundance and biomass are not easily quantified by multibeam echosounders, so we developed a new metric for delineating the gridded horizontal distribution of school thickness, and assessed the metric's efficacy by examining its correlation with mean volume backscattering strength derived from a calibrated 38 kHz split-beam echosounder (R = 0.67). We measured the distribution of fish school thickness around clusters of large concrete modules of an artificial reef using a multibeam echosounder, complemented with underwater video to aid species identification. The mean distribution of school thickness was mapped around the reef field with generalized additive mixed models. Model spatial predictions indicated schools aggregated around module clusters, rather than individual modules. Dynamic schools of fish in relatively shallow coastal waters ( 30 m) can be surveyed over 400,000 m 2 at 3 m s −1 in just 60 min. Portable multibeam echosounders are an accessible and valuable addition to quantifying the dynamic distributions of coastal fishes around features with high vertical relief.

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