Ecology of wild and cultured juvenile Trochus niloticus relevant to the use of juveniles for population enhancement

Castell Perez, Laura Lillana (1996) Ecology of wild and cultured juvenile Trochus niloticus relevant to the use of juveniles for population enhancement. PhD thesis, James Cook University.

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Natural stocks of the topshell Trochus niloticus L. (hereafter referred to as Trochus and trochus) have decreased significantly as a result of commercial fishing. This has led to the release of hatchery-reared juveniles being viewed as a potential tool to enhance populations. However, most experimental releases have resulted in low survival rates, with little known of the reasons for these results and how to improve them. The objectives of this study were to understand better the ecology of juvenile Trochus on the reef and to use this information to identify factors affecting their survival and growth.

Density, natural distribution, habitat characteristics and growth of juvenile Trochus and other gastropods on the intertidal reef flat at Orpheus Is., Australia, were sampled over a two-year period. Trochus between 1.5 mm and 62 mm shell width (SW) were found across the reef flat, but were most abundant in the middle section of the reef, 50-150 m off shore. There was no evidence of a size gradient. Mean density of Trochus was 0.178m⁻² in 1993 and 0.115 m⁻² in 1994. Trochus occurred in groups of 2-4 per m² more frequently than expected by random, but higher densities were very rarely observed. As Trochus size increased, there was a change from tending to occupy small rubble to occupying rock and coral bench, and from shallow to deeper pools. Growth rates estimated by progression of modal size classes ranged between 2.3 and 2.6 mm.month⁻¹.

The interaction between juvenile Trochus and three common invertebrate predators, the portunid crabs, Thalamita admete and T. stimpsoni, and the carnivorous gastropod, Thais tuberose, were examined. Interactions with crabs were studied in the laboratory. The size of trochus eaten increased with crab size, but larger crabs continued to eat the smallest trochus offered. Crabs also attacked relatively large trochus, up to 24 mm SW with low probability of success. The combined action of Th. admete and Th. stimpsoni may have a significant effect on the survival of Trochus < 20 mm SW, but mainly on smaller individuals (< 13 mm SW). Crabs responded to an increase in trochus density from 5 to 30 individuals.container⁻¹ by increasing their predation rate so that the proportion of trochus eaten after 24 h was significantly greater at higher density. When offered three patches with trochus at different densities (5, 15 and 30 trochus.patch⁻¹) in large raceways, crabs did not identify the patches of high density, but instead moved frequently around all patches. After 48h, the proportion eaten was not significantly different among the three densities. These observations of crab behaviour suggest there was no response to increased prey densities due to the crabs' mobile foraging behaviour.

Field distributions of Trochus, Turbo brunneus (another herbivorous gastropod) and Thais (a predatory gastropod) were compared, and the frequency of recently dead undamaged shells was used as an indication of mortality by non-crushing predators (e.g. Thais). Distributions of Trochus, Turbo and Thais overlapped and all were often found in close proximity. The proportions of undamaged shells that were recently killed was 10% for Trochus and 28% for Turbo. The behaviour of Trochus and Turbo (potential prey) to the presence of Thais was observed in laboratory experiments. Thais elicited a response from both prey species, but these differed considerably: Turbo showed a conventional flight escape response, whereas Trochus did not change speed but instead released white mucus. Cultured and wild Trochus showed the same response when exposed to Thais. Thais showed a strong preference for Turbo as prey, but the capture of Turbo was inhibited in water containing mucus released by Trochus. The mucus response of Trochus provides protection from predation by Thais.

The difficulty associated with finding trochus on the reef and how this may affect estimates of survival was examined in two experiments, one at Orpheus Is., Australia and another at Moso Is., Vanuatu, using flagged trochus (with a bright tag and easier to see) and unflagged trochus. Recapture of flagged trochus 2-3 days after the release was significantly higher than for unflagged trochus in both experiments, supporting the hypothesis that a significant proportion of trochus are overlooked by the observer. Mean sighting probability of trochus at Orpheus Is. was 0.69 for an average size of 23 mm SW and 0.81 at Moso Is. for an average size of 30 mm SW.

A series of field experiments directly related to seeding were conducted. Two field techniques were used: releasing trochus freely onto the reef and tethering trochus to rods hammered into the reef substratum. Free releases lasted between a few days to a maximum of 4 months and tether experiments lasted a maximum of eight days.

In two experiments I examined the effect of seeding density on survival and growth, one experiment at Orpheus Is. and one at Moso Is. The effect of seeding density on trochus growth was also examined at Orpheus Is. Survival was very similar between low and high seeding densities 2-3 days after the release, in both experiments. The effect of seeding density on survival after a few days was not significant at any site, but at Moso Is., significantly more trochus were recaptured in the deeper zone. One to four months after the release, survival was again not significantly different between seeding densities but growth rates of trochus released at low density were significantly greater than those of trochus released at high density. Individual growth rates ranged between 1- 5 mm.month⁻¹. The detrimental effect on growth rate suggests that trochus should be released at low rather than at high density.

In five experiments at Orpheus Is. I examined the effect of trochus size and habitat on survival. Two main patterns emerged with high consistency in the results. First, survival of larger trochus was always greater; however, survival estimates varied among experiments. Small trochus (4-12 mm SVV) and medium-large trochus were lost at a rate of 8-35% and 3-8% per day, respectively, over the first few days after the release. Second, trochus survival at different depth levels on the intertidal reef flat did not vary significantly. There was high variability among replicates, even five metres apart. Such variability is likely to be due to small scale differences in habitat characteristics which affect the distribution of predators and probability of trochus being detected. Analysis of trochus loss in a tether experiment showed that in 78% of cases where the number of trochus tethered to a rod decreased, only one trochus was lost at a time. This suggests that, after encountering and eating a trochus, predators did not remain in the area long enough to find another prey.

Mortality of cultured trochus immediately after release is very high but decreases with time, as is also the case for other species where cultured individuals are released for population enhancement. The results of this study emphasise the importance of increasing survival during that first period after release. The following procedures are likely to improve survival of released trochus:

1. It is better to release larger trochus. If possible, juveniles 20 mm SW or larger should be used in seeding reef environments.

2. It is better to spread trochus over a large section of the reef to reduce the risk of releasing them in an unfavourable area.

3. It is better to release trochus at low rather than high densities.

4. In monitoring survival through time, based on recapture rates, the probability of sighting a juvenile should be determined. This probability will depend on the size of trochus used and the characteristics of the habitat.

5. Great care should be given to the condition of the seed. Behaviour can be affected by disease or poor condition. Inappropriate behaviour (such as poor antipredator responses) could have important consequences on the susceptibility of trochus to attack by predators.

Item ID: 24099
Item Type: Thesis (PhD)
Keywords: density; distribution; ecology; growth; habitat; juveniles; mortality; Moso Island; Orpheus Island; portunid crabs; predator response; predator-prey interactions; prey density; prey selection; release; seeding density; size; survival; Thais tuberosa; Trochus niloticus; Turbo brunneus; Vanuatu
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Date Deposited: 11 Dec 2012 07:15
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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