Euan S. Harvey

Climate-driven regime shift of a temperate marine ecosystem

No turning back? Ecosystems over time have endured much disturbance, yet they tend to remain intact, a characteristic we call resilience. Though many systems have been lost and destroyed, for systems that remain physically intact, there is debate as to whether changing temperatures will result in shifts or collapses. Wernburg et al. show that extreme warming of a temperate kelp forest off Australia resulted not only in its collapse, but also in a shift in community composition that brought about an increase in herbivorous tropical fishes that prevent the reestablishment of kelp. Thus, many systems may not be resilient to the rapid climate change that we face. Science, this issue p. 169 Rapid warming tropicalizes a temperate kelp forest. Ecosystem reconfigurations arising from climate-driven changes in species distributions are expected to have profound ecological, social, and economic implications. Here we reveal a rapid climate-driven regime shift of Australian temperate reef communities, which lost their defining kelp forests and became dominated by persistent seaweed turfs. After decades of ocean warming, extreme marine heat waves forced a 100-kilometer range contraction of extensive kelp forests and saw temperate species replaced by seaweeds, invertebrates, corals, and fishes characteristic of subtropical and tropical waters. This community-wide tropicalization fundamentally altered key ecological processes, suppressing the recovery of kelp forests.

Estimation of reef fish length by divers and by stereo-video A first comparison of the accuracy and precision in the field on living fish under operational conditions

Abstract Using simultaneous measurements from an underwater stereo-video system the accuracy and precision of length estimates of reef fish made by three experienced diver scientists under field conditions is determined. The trial showed that under optimal conditions the divers’ estimates were very accurate ( mean error =0.87 cm ) but lacked precision ( mean S.D. =5.29 cm ) . The effects of the low precision are then demonstrated by using these field estimates to model the theoretical statistical power of the scientific divers to detect changes in the mean length of three species of fish from New Zealand coastal waters. The results suggest that the experienced diver scientists would have a much lower statistical power than stereo-video measurements to detect changes in the mean length, especially where low numbers of fish are recorded or where a research program aims to detect a change in the mean length of a fish population of 30% or less.

A comparison of underwater visual distance estimates made by scuba divers and a stereo-video system: implications for underwater visual census of reef fish abundance

Underwater visual census of reef fish by scuba divers is a widely used and useful technique for assessing the composition and abundance of reef fish assemblages, but suffers from several biases and errors. We compare the accuracy of underwater visual estimates of distance made by novice and experienced scientific divers and an under- water stereo-video system. We demonstrate the potential implications that distance errors may have on underwater visual census assessments of reef fish abundance. We also investigate how the accuracy and precision of scuba diver length estimates of fish is affected as distance increases. Distance was underestimated by both experienced (mean relative error =− 11.7%, s.d. = 21.4%) and novice scientific divers (mean relative error =− 5.0%, s.d. = 17.9%). For experienced scientific divers this error may potentially result in an 82% underestimate or 194% overestimate of the actual area censused, which will affect estimates of fish density. The stereo-video system also underestimated distance but to a much lesser degree (mean relative error =− 0.9%, s.d. = 2.6%) and with less variability than the divers. There was no correlation between the relative error of length estimates and the distance of the fish away from the observer.

The accuracy and precision of underwater measurements of length and maximum body depth of southern bluefin tuna (Thunnus maccoyii) with a stereo-video camera system

The accuracy and precision of in situ stereo-video measurements of the snout to fork length (SNFL) (range 830-1412 mm) and maximum body depth (MBD) (range 228-365 mm) of free-swimming southern bluefin tuna (SBT) (Thunnus maccoyii) were tested by filming live fish in sea cages immediately prior to harvest. Stereo-video measurements of the SNFL of 54 fish produced an average error of 1.72 mm (relative error of 0.16%), while an average error of 1.37 mm (relative error of 0.51%) was recorded for measurements of MBD from 47 fish.A procedure was developed to maximise the accuracy and precision of measurements of the SNFL and MBD from a single SBT over sequential images to avoid the underestimation of SNFL and overestimation of MBD due to sinusoidal changes in body form associated with fast swimming.The results demonstrate the potential of stereo-video systems to non-destructively make counts and measurements of tuna and other fish in both wild fisheries and mariculture situations, without the need to capture and handle them

A comparison of the accuracy and precision of measurements from single and stereo-video systems

Underwater tests using plastic silhouettes of fish were used to compare the accuracy and precision of measurements made with a single video camera system to those made from two stereo-video systems (one using digital camcorders, the other using Hi8 camcorders). Test measurements made across a variety of ranges and angles of silhouette orientation in the fields of view showed the length estimates from both the digital and Hi8 stereo-video systems were substantially more accurate and precise than those obtained by the single video camera system, and had the great advantage that the position (range and bearing) and orientation of a fish target could be measured directly. Measurements made with stereo-video were much less restricted by range and subject orientation than those made with single video. The data resulting from these trials are used to propose a set of guidelines to optimize the accuracy and precision of underwater measurements of fish length using single and stereo-video systems.

Hagfish predatory behaviour and slime defence mechanism

Hagfishes (Myxinidae), a family of jawless marine pre-vertebrates, hold a unique evolutionary position, sharing a joint ancestor with the entire vertebrate lineage. They are thought to fulfil primarily the ecological niche of scavengers in the deep ocean. However, we present new footage from baited video cameras that captured images of hagfishes actively preying on other fish. Video images also revealed that hagfishes are able to choke their would-be predators with gill-clogging slime. This is the first time that predatory behaviour has been witnessed in this family, and also demonstrates the instantaneous effectiveness of hagfish slime to deter fish predators. These observations suggest that the functional adaptations and ecological role of hagfishes, past and present, might be far more diverse than previously assumed. We propose that the enduring success of this oldest extant family of fishes over 300 million years could largely be due to their unique combination of functional traits.

Differences in fish assemblages from different reef habitats at Hamelin Bay, south-western Australia

Differences in the diversity of fish species between granite and limestone reefs, as well as high- and low-relief limestone reefs, were investigated at Hamelin Bay, south-western Australia. It was found that there were significant differences in the presence and abundance of fish species between granite reefs and limestone reefs. Granite reefs were characterized by greater numbers of Coris auricularis (western king wrasse) and Parma mccullochi (common scalyfin), whereas limestone reefs had greater numbers of the fish species Odax cyanomelas (herring cale), Pempheris klunzingeri (rough bullseye) and Kyphosus sydneyanus (common buffalo bream). A significant difference in fish diversity was also found between high-relief and low-relief limestone reefs in the same area. More species were found on the high-relief reefs than low-relief reefs. Complementing differences in fish assemblages, significant differences were found in algal assemblages from the different habitats. This was mainly owing to a dominance of Ecklonia radiata on low-relief limestone reefs. Ecklonia radiata was less dominant on granite reefs and on high-relief limestone reefs, where there was a lower overall algal biomass and a higher total number of species.

Tropical herbivores provide resilience to a climate-mediated phase shift on temperate reefs

Climate-mediated changes to biotic interactions have the potential to fundamentally alter global ecosystems. However, the capacity for novel interactions to drive or maintain transitions in ecosystem states remains unresolved. We examined temperate reefs that recently underwent complete seaweed canopy loss and tested whether a concurrent increase in tropical herbivores could be maintaining the current canopy-free state. Turf-grazing herbivorous fishes increased in biomass and diversity, and displayed feeding rates comparable to global coral reefs. Canopy-browsing herbivores displayed high (~ 10,000 g 100 m(-2) ) and stable biomass between 2006 and 2013. Tropical browsers had the highest abundance in 2013 and displayed feeding rates approximately three times higher than previously observed on coral reefs. These observations suggest that tropical herbivores are maintaining previously kelp-dominated temperate reefs in an alternate canopy-free state by grazing turfs and preventing kelp reestablishment. This remarkable ecosystem highlights the sensitivity of biotic interactions and ecosystem stability to warming and extreme disturbance events.

Diversity and Composition of Demersal Fishes along a Depth Gradient Assessed by Baited Remote Underwater Stereo-Video

Background Continental slopes are among the steepest environmental gradients on earth. However, they still lack finer quantification and characterisation of their faunal diversity patterns for many parts of the world. Methodology/Principal Findings Changes in fish community structure and diversity along a depth gradient from 50 to 1200 m were studied from replicated stereo baited remote underwater video deployments within each of seven depth zones at three locations in north-eastern New Zealand. Strong, but gradual turnover in the identities of species and community structure was observed with increasing depth. Species richness peaked in shallow depths, followed by a decrease beyond 100 m to a stable average value from 700 to 1200 m. Evenness increased to 700 m depth, followed by a decrease to 1200 m. Average taxonomic distinctness △+ response was unimodal with a peak at 300 m. The variation in taxonomic distinctness Λ+ first decreased sharply from 50 to 300 m, then increased beyond 500 m depth, indicating that species from deep samples belonged to more distant taxonomic groups than those from shallow samples. Fishes with northern distributions progressively decreased in their proportional representation with depth whereas those with widespread distributions increased. Conclusions/Significance This study provides the first characterization of diversity patterns for bait-attracted fish species on continental slopes in New Zealand and is an imperative primary step towards development of explanatory and predictive ecological models, as well as being fundamental for the implementation of efficient management and conservation strategies for fishery resources.

Similarities between Line Fishing and Baited Stereo-Video Estimations of Length-Frequency: Novel Application of Kernel Density Estimates

Age structure data is essential for single species stock assessments but length-frequency data can provide complementary information. In south-western Australia, the majority of these data for exploited species are derived from line caught fish. However, baited remote underwater stereo-video systems (stereo-BRUVS) surveys have also been found to provide accurate length measurements. Given that line fishing tends to be biased towards larger fish, we predicted that, stereo-BRUVS would yield length-frequency data with a smaller mean length and skewed towards smaller fish than that collected by fisheries-independent line fishing. To assess the biases and selectivity of stereo-BRUVS and line fishing we compared the length-frequencies obtained for three commonly fished species, using a novel application of the Kernel Density Estimate (KDE) method and the established Kolmogorov–Smirnov (KS) test. The shape of the length-frequency distribution obtained for the labrid Choerodon rubescens by stereo-BRUVS and line fishing did not differ significantly, but, as predicted, the mean length estimated from stereo-BRUVS was 17% smaller. Contrary to our predictions, the mean length and shape of the length-frequency distribution for the epinephelid Epinephelides armatus did not differ significantly between line fishing and stereo-BRUVS. For the sparid Pagrus auratus, the length frequency distribution derived from the stereo-BRUVS method was bi-modal, while that from line fishing was uni-modal. However, the location of the first modal length class for P. auratus observed by each sampling method was similar. No differences were found between the results of the KS and KDE tests, however, KDE provided a data-driven method for approximating length-frequency data to a probability function and a useful way of describing and testing any differences between length-frequency samples. This study found the overall size selectivity of line fishing and stereo-BRUVS were unexpectedly similar.