scholarly journals The Effect of the Decorator Crab Notomithrax Minor on Cultivated Greenshell Mussel Spat (Perna Canaliculus) Survival, Growth, and Byssal Characteristics

2021 ◽  
Author(s):  
◽  
Irene Van de Ven
Keyword(s):  
Retention Rate ◽  
Experimental Testing ◽  
Preliminary Evidence ◽  
Size Classes ◽  
Perna Canaliculus ◽  
Large Size ◽  
Field Investigations ◽  
Attachment Strength ◽  
Decorator Crab ◽  
Financial Losses

<p>The decorator crab Notomithrax minor is common on Greenshell mussel (Perna canaliculus) farms in the Marlborough Sounds, New Zealand. Individuals in the Greenshell mussel industry have suggested that the presence of N. minor, found on mussel lines, is related to substantial losses of Greenshell mussel spat. Laboratory and field investigations were used to assess the effect of N. minor presence on the retention and productivity of Greenshell musselTM spat. Specific consideration was given to predation pressure and induced anti-predator defenses, both of which can cause financial losses to mussel farmers. High (12 crabs/cage-1) and low (3 crabs/cage-1) densities of large (males: >20mm, females: >15mm TCW) and medium (males: 15-20mm, females: 10-15mm TCW) decorator crabs were placed in cages on commercial Greenshell mussel farm droppers at two sites in the Pelorus Sound. The byssal characteristics, spat retention rate and spat shell length were assessed at 8 and 11 weeks after trial initiation. Greenshell mussel density on the experimental droppers decreased significantly when medium and high densities of the decorator crabs (N. minor) were introduced. N. minor presence induced the remaining Greenshell mussel spat to produce more and thicker byssus threads which consequently lead to increased mussel attachment. The decrease in retention rate and the increase in mussel attachment strength were more pronounced in small recently seeded spat. Laboratory experiments to assess the consumption rate of small (≤5mm) Greenshell mussel spat by decorator crabs showed that mussel consumption by N. minor peaked at 56.43 (± 13.02 (95% C.I.)) crab-1 hr-1, however the rate of consumption decreased significantly over the duration of the three day trial. N. minor prey size preference was also assessed using Laboratory trials; crabs were offered 4 size classes of mussels (small (<5 mm), small-medium (5-10 mm), medium-large (10-15 mm), large (>15 mm) simultaneously. Female crabs consumed more mussels in the <5 mm and 5-10 mm size classes than in the two larger mussel size classes (10-15 mm and >15 mm), whereas the male crabs showed a numerical preference for mussel spat in the small-medium and medium-large size classes. This study provides preliminary evidence that the decorator crab N. minor is a previously overlooked and under-estimated threat to the Greenshell Mussel industry in the Marlborough Sounds that deserves closer scrutiny and experimental testing.</p>

scholarly journals The Effect of the Decorator Crab Notomithrax Minor on Cultivated Greenshell Mussel Spat (Perna Canaliculus) Survival, Growth, and Byssal Characteristics

2021 ◽  
Author(s):  
◽  
Irene Van de Ven
Keyword(s):  
Retention Rate ◽  
Experimental Testing ◽  
Preliminary Evidence ◽  
Size Classes ◽  
Perna Canaliculus ◽  
Large Size ◽  
Field Investigations ◽  
Attachment Strength ◽  
Decorator Crab ◽  
Financial Losses

<p>The decorator crab Notomithrax minor is common on Greenshell mussel (Perna canaliculus) farms in the Marlborough Sounds, New Zealand. Individuals in the Greenshell mussel industry have suggested that the presence of N. minor, found on mussel lines, is related to substantial losses of Greenshell mussel spat. Laboratory and field investigations were used to assess the effect of N. minor presence on the retention and productivity of Greenshell musselTM spat. Specific consideration was given to predation pressure and induced anti-predator defenses, both of which can cause financial losses to mussel farmers. High (12 crabs/cage-1) and low (3 crabs/cage-1) densities of large (males: >20mm, females: >15mm TCW) and medium (males: 15-20mm, females: 10-15mm TCW) decorator crabs were placed in cages on commercial Greenshell mussel farm droppers at two sites in the Pelorus Sound. The byssal characteristics, spat retention rate and spat shell length were assessed at 8 and 11 weeks after trial initiation. Greenshell mussel density on the experimental droppers decreased significantly when medium and high densities of the decorator crabs (N. minor) were introduced. N. minor presence induced the remaining Greenshell mussel spat to produce more and thicker byssus threads which consequently lead to increased mussel attachment. The decrease in retention rate and the increase in mussel attachment strength were more pronounced in small recently seeded spat. Laboratory experiments to assess the consumption rate of small (≤5mm) Greenshell mussel spat by decorator crabs showed that mussel consumption by N. minor peaked at 56.43 (± 13.02 (95% C.I.)) crab-1 hr-1, however the rate of consumption decreased significantly over the duration of the three day trial. N. minor prey size preference was also assessed using Laboratory trials; crabs were offered 4 size classes of mussels (small (<5 mm), small-medium (5-10 mm), medium-large (10-15 mm), large (>15 mm) simultaneously. Female crabs consumed more mussels in the <5 mm and 5-10 mm size classes than in the two larger mussel size classes (10-15 mm and >15 mm), whereas the male crabs showed a numerical preference for mussel spat in the small-medium and medium-large size classes. This study provides preliminary evidence that the decorator crab N. minor is a previously overlooked and under-estimated threat to the Greenshell Mussel industry in the Marlborough Sounds that deserves closer scrutiny and experimental testing.</p>

scholarly journals An external tag for fish: tagging effects in different fish size classes and its influence on growth performance and hematology of Lophiosilurus alexandri (Siluriformes: Pseudopimelodidae)

2019 ◽  
Vol 17 (1) ◽  
Author(s):  
Túlio P. Boaventura ◽  
Viviane S. B. Gil ◽  
Camila S. B. Gil ◽  
Anderson M. Peres ◽  
Ronald K. Luz
Keyword(s):  
Growth Performance ◽  
Retention Rate ◽  
Hematological Parameters ◽  
Size Classes ◽  
Tag Retention ◽  
Large Size ◽  
Large Size Class ◽  
Survival And Growth ◽  
Hematology Parameters ◽  
Animal Size

ABSTRACT This study tested an external tag for juveniles of Lophiosilurus alexandri and measured the effects of tagging in different size classes of fishes. Experiment 1 evaluated the retention rate and influence of the tag on survival and growth of three hundred fishes divided Small, Medium and Large size classes. After 90 days of experiment the fishes had 100% survival and the tag had a higher retention rate for animals of the Large size class. Experiment 2 evaluated the tag’s influence on hematology parameters of forty-two fishes at 5, 10 and 30 days post-tagging. In this experiment both tagged and untagged animals experienced 9.4% mortality. The hematocrit was higher on the 30th day for tagged animals than for untagged animals. No difference was observed for leukocytes, plasma protein, erythrocytes and mean corpuscular volume. It was concluded from Experiment 1 that the tag reduced growth performance when applied to small sized juvenile L. alexandri, and that the tag retention rate increased with increasing animal size. Among the hematological parameters studied for juveniles on the Experiment 2 the tag only influenced the hematocrit parameters, which shows that the tag can be used without considerable influence on the hematological parameters of juvenile L. alexandri.

Diet, feeding behaviour and habitat utilisation of the blue stingray Dasyatis chrysonota (Smith, 1828) in South African waters

2003 ◽  
Vol 54 (8) ◽  
pp. 957 ◽  
Author(s):  
David A. Ebert ◽  
Paul D. Cowley
Keyword(s):  
Surf Zone ◽  
Stomach Contents ◽  
Size Class ◽  
Behaviour Pattern ◽  
Medium Size ◽  
Secondary Importance ◽  
Size Classes ◽  
Important Prey ◽  
Large Size ◽  
Large Size Class

Analysis of stomach contents for Dasyatis chrysonota revealed that diet varied with size and habitat. The diet of all size classes in the surf zone was comprised primarily of Callianassa spp., Donax spp. and unidentified polychaete species. The medium and large size classes fed primarily on Donax spp., whereas the very large size class fed mainly on Callianassa spp. Polychaetes were of secondary importance as prey for the medium size class. The diet of D. chrysonota in the nearshore zone consisted mainly of Balanoglossus capensis and Callianassa spp. Balanoglossus capensis decreased from an index of relative importance (IRI) of 75.3% for the medium size class to 59.9% for the very large size class, whereas Callianassa spp. increased from 22.8% to 39.4% between the medium and the very large size classes. The offshore zone was the only area in which small size class D. chrysonota were caught. The diet of these small D. chrysonota was primarily polychaetes and amphipods. Polychaetes increased in importance in the medium size class, but declined in each successively larger size class. Conversely, Pterygosquilla armata capensis became the single most important prey item for the very large size class, comprising an IRI of 50.9%. The behaviour pattern used by D. chrysonota to locate and extract prey is described.

scholarly journals Evaluation of ReachOut.com, an Unstructured Digital Youth Mental Health Intervention: Prospective Cohort Study

10.2196/21280 ◽  
2020 ◽  
Vol 7 (10) ◽  
pp. e21280
Author(s):  
Bianca Lorraine Kahl ◽  
Hilary May Miller ◽  
Kathryn Cairns ◽  
Hayley Giniunas ◽  
Mariesa Nicholas
Keyword(s):  
Mental Health ◽  
Mental Health Services ◽  
Young People ◽  
Health Services ◽  
Retention Rate ◽  
Well Being ◽  
Evidence Base ◽  
Preliminary Evidence ◽  
Health Intervention ◽  
Mental Health Intervention

Background Young people experience a disproportionate burden associated with mental illness that Australia’s mental health care system is ill-equipped to handle. Despite improvements in the provision of mental health services, the rates of service utilization among young people remain suboptimal, and there are still considerable barriers to seeking help. Digital mental health services can overcome a number of barriers and connect young people requiring support; however, the evidence base of digital interventions is limited. Objective The aim of this study is to examine the effectiveness of a brief, self-directed, unstructured digital intervention, ReachOut.com (hereafter ReachOut), in reducing depression, anxiety, stress, and risk of suicide. Methods A cohort of 1982 ReachOut users participated in a 12-week longitudinal study, with a retention rate of 81.18% (1609/1982) across the duration of the study. Participants completed web-based surveys, with outcome measures of mental health status and suicide risk assessed at 3 time points across the study period. Results The results demonstrated that over the 12-week study period, young people using ReachOut experienced modest yet significant reductions in symptoms of depression, anxiety, and stress. Significant, albeit modest, reductions in the proportion of participants at high risk of suicide were also observed. Conclusions The findings of this research provide preliminary evidence of the promise of an unstructured digital mental health intervention, ReachOut, in alleviating symptoms of mental ill-health and promoting well-being in young people. These findings are particularly important given that digital services are not only acceptable and accessible but also have the potential to cater to the diverse mental health needs of young people at scale, in a way that other services cannot.

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