Predators and habitat association of post-settlement snapper (Chrysophrys auratus)

Abstract

Structured habitats play an important nursery role during the crucial early juvenile or post-settlement stages of many fish species. Predominantly, the utility of structured habitats to juvenile fish is thought to be associated with the provisioning of food or as a refuge from predation. Although snapper (Chrysophrys auratus) in New Zealand also have a strong affinity for structured habitats during their post-settlement phase, their predators are unknown as is the role of predation in determining habitat association. Here the authors investigated potential predators of post-settlement snapper by remotely observing interactions of restrained post-settlement snapper with potential predators and investigating the diet of potential predators. They also conducted tank experiments with a potential predator, both with and without the presence of structure. Restrained snapper were infrequently approached by predators, but two new nocturnal predators were identified. No snapper were observed in the diet of potential predators, although two piscivores were identified as potential candidates. No predation occurred during tank experiments, but there was a non-significant indication that under threat of predation post-settlement snapper may use habitat when it is present and aggregate together when it is not. The findings suggest that the pulsed nature of predation may have made it difficult to observe given the methods employed and that the threat of predation may be sufficient to drive the habitat selection of post-settlement snapper. Investigating the significance of predation via methods that do not require direct observations may therefore be more appropriate given this context.

Keywords: Pagrus auratus; biogenic nursery habitat; juvenile fish; nocturnal predators; post-settlement snapper; predation refuge.

FIGURE 1

Map of Whangarei Harbour showing the locations where restrained post‐settlement snapper were deployed on artificial seagrass units (ASUs), and the location of set nets to capture potential predators of post‐settlement snapper. New Zealand and the location of Whangarei Harbour inset. , Artificial Seagrass Units; , Set nets; , Intertidal; , Land

FIGURE 2

Vertical and horizontal diagram views of the remote camera and infrared illuminator set‐up used to observe restrained post‐settlement snapper. Note that the camera position changed depending on whether a daytime or night‐time deployment was being conducted, but was always 64 cm from the restrained post‐settlement snapper

FIGURE 3

Screenshot images of predators taken during night‐time deployments of restrained post‐settlement snapper (in plastic jar at centre of image). (a) Conger eel and (b) broad squid

FIGURE 4

Fish and invertebrate species and age categories (at different times of the day) observed by camera deployments conducted with restrained post‐settlement snapper. Species names not mentioned elsewhere: goatfish (Upeneichthys lineatus). , Day; , Dusk; , Night

FIGURE 5

Non‐metric multiple dimensional scaling (MDS) ordination of the presence of fish and invertebrate species and age categories (at different times of the day) observed by camera deployments conducted with restrained post‐settlement snapper. A biplot of Pearson correlations of contributing species/age groups with canonical axes is overlaid on the ordination (correlations >0.4 represented). Abbreviations as follows: 0 + _TRE = 0+ trevally, 0+ STY = 0+ spotty, GAR = piper, EGR = eagle ray, PS_SNA = post‐settlement snapper, A_SNA = adult snapper, CON = conger eel. , Day time; , Dusk time; , Night time

FIGURE 6

Diet composition (% volume) of fish species captured by set net from four sites in Whangarei Harbour. Numbers above each bar refer to the number of stomachs that contained at least some gut contents for each species. , Benthic Crustacea; , Bivalvia; , Gastropoda; , Polychaeta; , Algae/Seagrass; , Porifera; , Shell/sand/gravel; , Teleostei; , Tunicata; , Unidentified invertebrate

FIGURE 7

Non‐metric multiple dimensional scaling (MDS) ordination of behaviours expressed by post‐settlement snapper and kingfish predators [with and without the presence of physical structure provided by artificial seagrass units (ASUs)] as observed in tank experiment trials conducted at the the Northland Marine Research Centre (NMRC). Each data point is labelled with the year in which that particular experimental trial was conducted. A biplot of Pearson correlations of contributing behaviours (labels as per Table 2) with canonical axes is overlaid on the ordination (correlations >0.65 represented). , Control treatment; , ASU