Craig A. Radford

Location, location, location: finding a suitable home among the noise

While sound is a useful cue for guiding the onshore orientation of larvae because it travels long distances underwater, it also has the potential to convey valuable information about the quality and type of the habitat at the source. Here, we provide, to our knowledge, the first evidence that settlement-stage coastal crab species can interpret and show a strong settlement and metamorphosis response to habitat-related differences in natural underwater sound. Laboratory- and field-based experiments demonstrated that time to metamorphosis in the settlement-stage larvae of common coastal crab species varied in response to different underwater sound signatures produced by different habitat types. The megalopae of five species of both temperate and tropical crabs showed a significant decrease in time to metamorphosis, when exposed to sound from their optimal settlement habitat type compared with other habitat types. These results indicate that sounds emanating from specific underwater habitats may play a major role in determining spatial patterns of recruitment in coastal crab species.

Pressure and particle motion detection thresholds in fish: a re-examination of salient auditory cues in teleosts

SUMMARY The auditory evoked potential technique has been used for the past 30 years to evaluate the hearing ability of fish. The resulting audiograms are typically presented in terms of sound pressure (dB re. 1 μPa) with the particle motion (dB re. 1 m s−2) component largely ignored until recently. When audiograms have been presented in terms of particle acceleration, one of two approaches has been used for stimulus characterisation: measuring the pressure gradient between two hydrophones or using accelerometers. With rare exceptions these values are presented from experiments using a speaker as the stimulus, thus making it impossible to truly separate the contribution of direct particle motion and pressure detection in the response. Here, we compared the particle acceleration and pressure auditory thresholds of three species of fish with differing hearing specialisations, goldfish (Carassius auratus, weberian ossicles), bigeye (Pempheris adspersus, ligamentous hearing specialisation) and a third species with no swim bladder, the common triplefin (Forstergyian lappillum), using three different methods of determining particle acceleration. In terms of particle acceleration, all three fish species have similar hearing thresholds, but when expressed as pressure thresholds goldfish are the most sensitive, followed by bigeye, with triplefin the least sensitive. It is suggested here that all fish have a similar ability to detect the particle motion component of the sound field and it is their ability to transduce the pressure component of the sound field to the inner ear via ancillary hearing structures that provides the differences in hearing ability. Therefore, care is needed in stimuli presentation and measurement when determining hearing ability of fish and when interpreting comparative hearing abilities between species.

Behavioural response thresholds in New Zealand crab megalopae to ambient underwater sound.

A small number of studies have demonstrated that settlement stage decapod crustaceans are able to detect and exhibit swimming, settlement and metamorphosis responses to ambient underwater sound emanating from coastal reefs. However, the intensity of the acoustic cue required to initiate the settlement and metamorphosis response, and therefore the potential range over which this acoustic cue may operate, is not known. The current study determined the behavioural response thresholds of four species of New Zealand brachyuran crab megalopae by exposing them to different intensity levels of broadcast reef sound recorded from their preferred settlement habitat and from an unfavourable settlement habitat. Megalopae of the rocky-reef crab, Leptograpsus variegatus, exhibited the lowest behavioural response threshold (highest sensitivity), with a significant reduction in time to metamorphosis (TTM) when exposed to underwater reef sound with an intensity of 90 dB re 1 µPa and greater (100, 126 and 135 dB re 1 µPa). Megalopae of the mud crab, Austrohelice crassa, which settle in soft sediment habitats, exhibited no response to any of the underwater reef sound levels. All reef associated species exposed to sound levels from an unfavourable settlement habitat showed no significant change in TTM, even at intensities that were similar to their preferred reef sound for which reductions in TTM were observed. These results indicated that megalopae were able to discern and respond selectively to habitat-specific acoustic cues. The settlement and metamorphosis behavioural response thresholds to levels of underwater reef sound determined in the current study of four species of crabs, enables preliminary estimation of the spatial range at which an acoustic settlement cue may be operating, from 5 m to 40 km depending on the species. Overall, these results indicate that underwater sound is likely to play a major role in influencing the spatial patterns of settlement of coastal crab species.

The contribution of the lateral line to ‘hearing’ in fish

SUMMARY In the underwater environment, sound propagates both as a pressure wave and as particle displacement, with particle displacement dominating close to the source (the nearfield). At the receptor level, both the fish ear and the neuromast hair cells act as displacement detectors and both are potentially stimulated by the particle motion component of sound sources, especially in the nearfield. A now common way to test ‘hearing’ in fish involves auditory evoked potentials (AEPs), with recordings made from electrodes implanted near the auditory brainstem. These AEP recordings are typically conducted in enclosed acoustic environments with the fish well within the nearfield, especially for lower frequencies. We tested the contribution of neuromast hair cells to AEP by first testing intact goldfish (Carassius auratus), then ablating their neuromasts with streptomycin sulphate — disabling superficial and canal neuromasts — and retesting the same goldfish. We performed a similar experiment where only the superficial neuromasts were physically ablated. At 100 and 200 Hz, there was a 10–15 dB increase in threshold after streptomycin treatment but no significant difference at higher frequencies. There was no difference in threshold in control fish or in fish that only had superficial neuromasts removed, indicating that the differential responses were driven by canal neuromasts. Taken together, these results indicate that AEP results at lower frequencies should be interpreted as multimodal responses, rather than as ‘hearing’. The results also suggest that in natural situations both the ear and lateral line likely play an integrative role in detecting and localising many types of ‘acoustic’ stimuli.

Turbine Sound May Influence the Metamorphosis Behaviour of Estuarine Crab Megalopae

It is now widely accepted that a shift towards renewable energy production is needed in order to avoid further anthropogenically induced climate change. The ocean provides a largely untapped source of renewable energy. As a result, harvesting electrical power from the wind and tides has sparked immense government and commercial interest but with relatively little detailed understanding of the potential environmental impacts. This study investigated how the sound emitted from an underwater tidal turbine and an offshore wind turbine would influence the settlement and metamorphosis of the pelagic larvae of estuarine brachyuran crabs which are ubiquitous in most coastal habitats. In a laboratory experiment the median time to metamorphosis (TTM) for the megalopae of the crabs Austrohelice crassa and Hemigrapsus crenulatus was significantly increased by at least 18 h when exposed to either tidal turbine or sea-based wind turbine sound, compared to silent control treatments. Contrastingly, when either species were subjected to natural habitat sound, observed median TTM decreased by approximately 21–31% compared to silent control treatments, 38–47% compared to tidal turbine sound treatments, and 46–60% compared to wind turbine sound treatments. A lack of difference in median TTM in A. crassa between two different source levels of tidal turbine sound suggests the frequency composition of turbine sound is more relevant in explaining such responses rather than sound intensity. These results show that estuarine mudflat sound mediates natural metamorphosis behaviour in two common species of estuarine crabs, and that exposure to continuous turbine sound interferes with this natural process. These results raise concerns about the potential ecological impacts of sound generated by renewable energy generation systems placed in the nearshore environment.

Bubbled waters: The noise generated by underwater breathing apparatus

Underwater breathing apparatus (UBA) has played a vital role in the study of aquatic environments, and is commonly used in visual census of mobile aquatic animals. The possibility of artifacts arising from diver presence and from the noise produced by UBA have long been recognised but not systematically studied. Here we analyse the noise produced by the three types of UBA used for research; self-contained underwater breathing apparatus (SCUBA), semi-enclosed circuit re-breather (SECR), and fully enclosed circuit re-breather (FECR) systems. There were significant differences in the source levels (SL) produced by the different UBA for both mean SL (p < 0.001) and mean peak SL (p < 0.001). SCUBA produced the most noise followed by SECR and FECR (161 ± 1, 131 ± 2, and 108 ± 1 dB re 1 μPa at 1 m, ±S.E.). Much of the sound produced by all three UBA was at low frequencies (<200 Hz), the range in which the hearing organs of fish and decapod crustaceans are most sensitive. Calculations indicated that the UBA are likely to be detectable by fishes at considerable distances depending on natural ambient noise levels.

Snapper (Chrysophrys auratus): a review of life history and key vulnerabilities in New Zealand

Snapper (Chrysophrys auratus) is an important coastal fish species in New Zealand for a variety of reasons, but the large amount of research conducted on snapper has not been reviewed. Here, we review life history information and potential threats for snapper in New Zealand. We present information on snapper life history, defining stages (eggs and larvae, juvenile and adult), and assess potential threats and knowledge gaps. Overall we identify six key points: 1. post-settlement snapper are highly associated with certain estuarine habitats that are under threat from land-based stressors. This may serve as a bottleneck for snapper populations; 2. the largest knowledge gaps relate to the eggs and larvae. Additional knowledge may help to anticipate the effects of climate change, which will likely have the greatest influence on these early life stages; 3. ocean acidification, from land-based sources and from climate change, may be an important threat to larval snapper; 4. a greater understanding of population connectivity would improve certainty around the sustainability of fishery exploitation; 5. the collateral effects of fishing are likely to be relevant to fishery productivity, ecosystem integrity and enduser value; 6. our understanding of the interrelationships between snapper and other ecosystem components is still deficient.

Can larval snapper, Pagrus auratus, smell their new home?

The ability to find a suitable settlement habitat after a pelagic larval period represents a significant challenge to marine settlement-stage larvae, and the mechanisms by which they achieve this are poorly understood. There is good evidence that olfactory cues are used by some coral reef fish larvae to locate suitable settlement habitats; however, the same understanding is lacking for marine temperate fish. Here we show for the first time that the larvae of an important commercial and recreational marine temperate fish, Pagrus auratus, can use olfactory cues to orient to appropriate settlement habitat. Using pairwise choice experiments, naive hatchery reared fish were offered water collected from a range of habitats in the Kaipara Harbour, an important nursery area for P. auratus. Larvae selected to swim towards water taken from over seagrass beds, their preferred settlement habitat, than water taken from the harbour entrance, Asian date mussel habitat, artificial seawater or artificial seawater in which seagrass had been soaked. The preference by the fish for water from the seagrass habitat over artificial seawater in which seagrass had been soaked strongly suggests that chemical cues from sources other than seagrass, such as from prey or conspecifics present in the seagrass habitat, may also be involved.

Variation in the growth of larval and juvenile snapper, Chrysophrys auratus (Sparidae)

For many fish species, growth and mortality of larvae are closely coupled, with faster-growing larvae generally experiencing higher survivorship in the plankton, which may lead to higher recruitment. Using back-calculated growth trajectories derived from otolith increments we used the modified Fry model to estimate the growth rate of larvae and early juveniles of the commercially important sparid, Chrysophrys auratus, at four sites around northern New Zealand. Back-calculated growth rates were used to test the hypothesis that fish with a short pelagic larval duration (≤20 days) grew faster than did fish with a long pelagic larval duration (>24 days) during both the larval and juvenile periods. At three of the four sites, fish with a short larval duration grew significantly faster during the larval period, and these larvae generally continued to have a larger size-at-age as juveniles up to 70-day-old. Growth rates for both the larval and early juvenile period were also found to vary significantly among the four sites and were found to be unrelated to differences in water temperature. Localised variation in early growth of C. auratus among sites may be important in helping explain differences in their contribution to the recruitment to C. auratus populations.

The cumulative effect on sound levels from multiple underwater anthropogenic sound sources in shallow coastal waters

Summary 1. Underwater anthropogenic sound levels in the ocean are increasing, and the evidence for impacts of sound on marine life is overwhelming. Currently, regulatory bodies require emitted sound from marine renewable energy projects to be assessed in order to infer potential impacts on marine life. Geometric spreading models are sometimes used to assess likely ecological impacts of anthropogenic sound by estimating the propagation of underwater sound without in situ verification. 2. This study investigated the propagation of anthropogenic sound in shallow waters (25– 45 m deep) from a single monopole (a single loudspeaker) and twin monopole (two loudspeakers) sound sources replaying tidal turbine sound to approximate anthropogenic sound sources in this environment. 3. Greater sound levels at 100–800, 801–2000 and 2001–20 000 Hz were observed at all observation distances from the sound source for the twin monopole compared to the single monopole. These results suggest that the sound from multiple anthropogenic sound sources installed in one location is likely to be cumulative in the environment where their sound fields overlap. 4. Sound intensities observed in both the single monopole and twin monopole experiments for 0� 1–20 kHz were higher than estimates derived from standard geometric spreading models for underwater sound by 3–41 dB across all measured distances from the source. The greatest difference from the theoretical spherical spreading model (41 dB) occurred at the site most distant from the twin monopole sound source, that is, 5 km. 5. Synthesis and applications. The results show that geometric models used by some regulatory bodies may be underestimating the spatial extent to which the anthropogenic underwater sound may be propagating and creating potential ecological impacts. Based on field measurements, we have presented an alternative model which should assist regulatory agencies to better estimate and manage ecological impacts from anthropogenic underwater sound.