Science corner: latest fish & fisheries research

[Visenvryheid]

Very interesting reading! So we got good at catching the big cob, and perhaps also better at catching the leeries (Anecdotally from the old timers the leeries were in all the haunts they are now, in the old days, but in much much larger numbers of far larger fish in the old days. The galjoen has been hammered and we have fished out the red and black steenbras.

[FishStyx]

All the more reason to keep catch reports flowing on responsible social media like UA. 

[John F]

Growth Limitation of Marine Fish by Low Iron Availability in the Open Ocean


* It is well-established that phytoplankton growth can be limited by the vanishingly low concentrations of dissolved iron found in large areas of the open ocean;


* However, the availability of iron is not typically considered an important factor in the ecology of marine animals, including fish;


* Observations that show that the iron contents of lower trophic level organisms in iron-limited regions can be an order of magnitude less than the iron contents of most fish were compiled;


* Although this shortfall could theoretically be overcome if iron assimilation rates were very high in fish, observations suggest this is not the case, consistent with the high recommended iron contents for mariculture feed;


* In addition, two occurrences among fish living in iron-poor regions that would conceivably be beneficial given iron scarcity are highlighted: the absence of hemoglobin in Antarctic icefish, and the anadromous life history of salmon;


* Based on these multiple lines of evidence, it is suggested that the iron content of lower trophic level organisms can be insufficient to support many fish species throughout their life cycles in iron-poor
oceanic regions;


* A global satellite-based estimate of fishing effort was used to show that relatively little fishing activity occurs in high nitrate low chlorophyll (HNLC) regions, the most readily identified iron-poor domains of the ocean, particularly when compared to satellite-based estimates of primary production and the observed mesozooplankton biomass in those waters;


* The low fishing effort is consistent with a low abundance of epipelagic fish in iron-limited regions, though other factors are likely to contribute as well;


* The results imply that the importance of iron nutrition extends well beyond plankton and plays a role in the ecology of large marine animals.



pdf available.
Reference:
Galbraith, E. D., P. Le Mézo, G. S. Hernandez, D. Bianchi & D. Kroodsma (2019). Growth limitation of marine fish by low iron availability in the open ocean. Frontiers in Marine Science, 6: 509. doi: 10.3389/fmars.2019.00509

[John F]

Life history, distribution and seasonal movements of a threatened South African endemic seabream, Chrysoblephus gibbiceps
* The red stumpnose Chrysoblephus gibbiceps (Sparidae) is a South African endemic seabream that has been severely depleted by fishing.


* A total of 678 C. gibbiceps were sampled by line and trawl fishing for a study of their morphology, age, growth, reproduction and diet. In addition, catch records from three time-periods since 1897 were interrogated for distribution patterns and movement behaviour.


* The sex ratio was 1.5:1 in favour of males. The length–weight relationship, W = 9.32 × 10−5 LF 2.811, indicates hypoallometric growth.


* The bulbous head of large males is a secondary sexual characteristic.


* A von Bertalanffy growth model was fitted to age data obtained using otolith analyses: LF = 429.9 × (1 – e−0.113(t – (−3.799)).


* The maximum recorded age was 48 years, which is among the highest in seabreams, though the species matures after only 3 years.


* The gonadosomatic index (GSI) of ripe females (4.5%) was comparatively low, and the two-month-long early-summer spawning season was short compared with that of other sympatric seabreams.


* The very low GSI of ripe males (1.6%) suggests courtship battles and polygamy.


* The species feeds over low-profile reefs and consolidated sediments.


* The principal prey are ophiuroids.


* Although its trophic level is 3.7, C. gibbiceps has a low-nutrition diet.


* Historical and current catch data confirm a distribution from Cape Point (southeastern Atlantic) to southern KwaZulu-Natal Province (western Indian Ocean).


* There is evidence for localised migratory patterns, now partly lost due to severe population depletion.


* Whereas protogyny and resident behaviour have been suggested as traits that render seabreams vulnerable to fishing pressure, C. gibbiceps is a shoaling gonochorist that has collapsed due to fishing.





pdf available.

Reference:Attwood, C. G., M. E. Dawson, S. E. Kerwath & C. Wilke (2019). Life history, distribution and seasonal movements of a threatened South African endemic seabream, Chrysoblephus gibbiceps. African Journal of Marine Science, 41 (4): 395-411.

[FishStyx]

Thanks Marcos... And thank you for keeping this thread going.


The sex ratio... I wonder if that's actual or of those caught... Any idea?

[John F]

The sex ratio is of those 678 fish sampled. It is usually assumed that the sample is random (unless there's evidence that points otherwise), and thus indicative of the actual situation.

That's why sample size is crucial... and you will always hear biologists (or any scientist for that matter!), cry over sample size and claiming more studies are needed (well, also need to pay the bills heheheheh)

[FishStyx]

I was wondering if males are just more stupid (and prone to getting conned by anglers) than females... 

[John F]

Well, usually males of any species are more stupid ...  but don't really think it matter in this case...

However, in some species (the hammerhead shark for example), are notorious for large aggregation of females .... and these could potentially be risky in terms of wiping out females or males...

[John F]

Spatial trophic variability of a coastal apex predator, the giant trevally Caranx ignobilis, in the western Indian Ocean
* Top predators have substantial downstream effects on the structure, function, and resilience of ecosystems.

* The influence of top predators on an ecosystem can vary if they occur within multiple habitat types and have a wide niche breadth due to spatiotemporal changes in diet.

* The spatial patterns in trophic position and niche width for an economically important reef-associated fishery species, the giant trevally Caranx ignobilis were examined.
* Four localities in the western Indian Ocean representing different habitats: coral atolls, coastal reefs, and granitic islands were sampled.

* Isotopic ratios of carbon (13C/12C) and nitrogen (15N/14N) were analyzed,  compound-specific amino acid stable isotope analysis (AA-CSIA) performed to control for baseline nitrogen variation.

* The analysis of 12 juveniles and 43 adults revealed wide variation in trophic niche breadth between sampling sites and an offshore to coastal gradient in carbon that drove niche distinctiveness between localities.

* Niche width patterns suggestive of ontogenetic changes in diet and habitat utilization and larger niche sizes at the oceanic island sites than the coastal site were observed.

* Trophic position estimates ranged from 3.5−5, expanding the trophic range of C. ignobilis relative to previous studies using AA-CSIA and placing it at the equivalent trophic level as many predatory sharks.

* The study corroborates prior evidence that C. ignobilis is an important apex predator in reef and island ecosystems.

* It is shown how evaluating spatiotemporal components of trophic ecology of marine predators is critical for characterizing their functional
role and ecosystem influence, allowing for targeted conservation efforts.

pdf available.

Reference:Glass, J. R., R. Daly, P. D. Cowley & D. M. Post (2020). Spatial trophic variability of a coastal apex predator, the giant trevally Caranx ignobilis, in the western Indian Ocean. Marine Ecology Progress Series, 641: 195-208.

[Chancer]

And now in English? 

[John F]

hahahah here it goes:

* GTs are important predators (same trophic level as many sharks) that take on a large variety of prey items;

* Juvenile GTs prey on different items comparing with adults (the diet changes with age)

* GTs from oceanic islands have a slightly different diet comparing to those in coastal (=continental) areas!

[MIKE PIKE]

Interesting info , thx Marcos ... will have to change my tactics when at the sea again ...

[FishStyx]

will have to change my tactics when at the sea again ...

Are we ever going to be allowed back to the ocean 


Thanks Marcos... The translation was useful  ... They really are apex predators... Fight clubbers of the reef, as opposed to Sharks, which IMHO are opportunists of the reef. That's what makes geets such an awesome target.

[John F]

Ecosystem service values of sediment generation and entrapment by marginal coral reefs at Sodwana Bay, South Africa


* Coral reefs provide a multitude of goods and services, some of which are difficult to value due to their intangible nature and the absence of markets to ascribe their relative worth;



* The coral reefs of Sodwana Bay on the northeast coast of South Africa provide several ecological goods and services, of which only two are considered here: namely, sediment generation and sediment entrapment;



* Both are deemed essential to the functioning of the Sodwana Bay economy;



* The replacement-cost method was used to estimate the annual financial cost of sediment provided to the study area if it were replaced by dredging;


* Sediment generation by the coral reefs was valued at R2.6–R4.8 million, and sediment entrapment valued at R71.8–R84.6 million, totalling between R74.4 million and
R89.4 million (≈$5.6–$6.7 million, at R13.38/US$1) per year;



PDF available.


Reference:
Laing, S. C. S, M. H Schleyer & J. K. Turpie (2020). Ecosystem service values of sediment generation and entrapment by marginal coral reefs at Sodwana Bay, South Africa. African Journal of Marine Science, 42 (2): 199-207.

[John F]

Giant trevally (Caranx ignobilis) of Hawaiian Islands can live 25 years


* Giant trevally (Caranx ignobilis) are an iconic reef fish of the Indo-Pacific, and yet knowledge of their life history is incomplete.


* The species is common in tropical and subtropical waters ranging from the far western Indian Ocean off South Africa to the central Pacific Ocean,
where the species attains its greatest recorded size in Hawaii.


* Despite their broad range, importance as a top predator and attraction among anglers, the age and longevity of giant trevally remain largely undescribed and there are concerns that giant trevally are being fished down in the Hawaiian Islands.


* The most comprehensive age and growth study was performed in the Hawaiian Islands, but the work was limited to early growth observations.


* Although a study of daily increments was limited to fish less than maximum size, a model used to describe the growth characteristics was extrapolated to estimate that maximum age may exceed 20 years.


* In this study, otoliths of giant trevally from the Hawaiian Islands were investigated for annual growth zones, and the validity of the age estimates was tested and validated with bomb 14C dating.


* Revised growth parameters indicate giant trevally grow rapidly in the Hawaiian Islands and can attain an age of 25 years.



pdf available.


Reference:


Andrews, A. H. (2020). Giant trevally (Caranx ignobilis) of Hawaiian Islands can live 25 years. Marine & Freshwater Research, 71(10) 1367-1372.

[FishStyx]

Giant trevally (Caranx ignobilis) of Hawaiian Islands can live 25 years


* Giant trevally (Caranx ignobilis) are an iconic reef fish of the Indo-Pacific, and yet knowledge of their life history is incomplete.


* The species is common in tropical and subtropical waters ranging from the far western Indian Ocean off South Africa to the central Pacific Ocean,
where the species attains its greatest recorded size in Hawaii.


* Despite their broad range, importance as a top predator and attraction among anglers, the age and longevity of giant trevally remain largely undescribed and there are concerns that giant trevally are being fished down in the Hawaiian Islands.


* The most comprehensive age and growth study was performed in the Hawaiian Islands, but the work was limited to early growth observations.


* Although a study of daily increments was limited to fish less than maximum size, a model used to describe the growth characteristics was extrapolated to estimate that maximum age may exceed 20 years.


* In this study, otoliths of giant trevally from the Hawaiian Islands were investigated for annual growth zones, and the validity of the age estimates was tested and validated with bomb 14C dating.


* Revised growth parameters indicate giant trevally grow rapidly in the Hawaiian Islands and can attain an age of 25 years.


* They are the most awesome fish in the ocean.


Reference:


Andrews, A. H. (2020). Giant trevally (Caranx ignobilis) of Hawaiian Islands can live 25 years. Marine & Freshwater Research, 71(10) 1367-1372.

There... Fixed it 

[REEFMAN]

Agree with Fishstyx!   

[John F]

"I would die to see one": A study to evaluate safety knowledge, attitude, and behavior among shark SCUBA divers



* Shark diving tourism is an activity that can contribute significantly to coastal economies, while also offering tremendous help to shark conservation efforts.

* Nevertheless, like any form of wildlife-based tourism, shark diving poses management challenges revolving around ethical and safety considerations.

* Safety in shark diving normally focuses on operational self-efficacy and adherence to shark diving codes of conduct to prevent incidents such as shark bites and to minimize ecological harm.

* However, safety issues in shark diving can arise from personal choices to exceed standard certification limits.


* Any detrimental results are capable of casting doubts on the sustainability of shark diving, thus jeopardizing its future as well as shark conservation.


* This study addressed compliance with shark diving codes of conduct and standard diving safety by examining the knowledge, attitude, and behavior of people who engage in free scuba diving with predatory sharks.


* The research made use of mixed methods of data collection, including interviews with shark divers at two popular shark diving destinations in Southeast Africa (n = 86) and an online questionnaire survey among shark divers (n = 89).


* The results showed that divers had positive attitudes towards sharks and shark diving.


* However, a notable proportion declared that they had exceeded certification limits and broken codes of conduct during shark diving.


* In particular, diving experience and being a professional diver were correlated significantly with poor safety attitudes and behavior.


* The results highlight the need to create an understanding among scuba divers of the connection between shark diving safety and conservation, including the negative implications of safety breaches, whether big or small, for the future of shark diving tourism and of sharks.


PDF available.


Reference:
Lucrezi, S., F. Bargnesi & F. Burman (2020). "I would die to see one": A study to evaluate safety knowledge, attitude, and behavior among shark SCUBA divers. Tourism in Marine Environments, 15 (3-4): 127-158.

[John F]

Maximum swimming speeds of sailfish and three other large marine predatory fish species based on muscle contraction time and stride length: a myth revisited


* Billfishes are considered to be among the fastest swimmers in the oceans.


* Previous studies have estimated maximum speed of sailfish and black marlin at around 35 m s⁻¹ but theoretical work on cavitation predicts that such extreme speed is unlikely.


* This study we investigated maximum speed of sailfish, and three other large marine pelagic predatory fish species, by measuring the twitch contraction time of anaerobic swimming muscle.


* The highest estimated maximum swimming speeds were found in sailfish (8.3±1.4 m s⁻¹), followed by barracuda (6.2±1.0 m s⁻¹), little tunny (5.6±0.2 m s⁻¹) and dorado (4.0±0.9 m s⁻¹); although size-corrected performance was highest in little tunny and lowest in sailfish.


* Contrary to previously reported estimates, the results suggest that sailfish are incapable of exceeding swimming speeds of 10-15 m s⁻¹, which corresponds to the speed at which cavitation is predicted to occur, with destructive consequences for fin tissues.


PDF available.


Reference:
Svendsen, M. B. S. et al. (2016). Maximum swimming speeds of sailfish and three other large marine predatory fish species based on muscle contraction time and stride length: a myth revisited. Biology Open, 5: 1415-1419

[John F]

Trends in shore-based angling effort determined from aerial surveys: a case study from KwaZulu-Natal, South Africa



* Understanding the spatiotemporal characteristics of effort is a critical component of managing fisheries.

* Recreational shore-angling is the largest sector of the linefishery and one of the primary recreational activities undertaken along the coast of KwaZulu-Natal (KZN), South Africa.


* The aim of this study was to make a reliable estimate of current total shore-based angling effort in KZN and to examine the spatial and temporal distribution of the effort.


* To do this, randomised monthly flights to count shore-anglers in an ‘instantaneous’ manner were conducted along the entire KZN coastline between January and December 2018, and then compared with similar aerial surveys conducted in 1994/95 and 2007/08.


* A total of 44 flights were undertaken, with 21 and 23 flights conducted along the north and south coasts, respectively.


* Ground-truthing revealed that the aerial counts of shore-anglers were 89% accurate.


* Angler effort was significantly higher over weekends and during good-weather days.


* Seasonality of shore-based angling effort showed that greatest effort occurred during the winter months (June to September), coinciding with the seasonal availability of popular angling species.


* More-developed stretches of the KZN coast with higher population densities and easier beach-access had the highest angling effort.


* Total annual angler effort was calculated to be 785 538 angler-days y–1, which represents a 22.9% decline from the estimate made in 1994/95 and a 6.9% decline from the estimate made in 2007/08.


* Spatial distribution of shore-based fishing effort was very similar in 2007/08 and 2018, with the major differences being a decrease in fishing effort at St Lucia and Maphelane, and a slight increase in effort on the ‘upper’ (northern) south coast during 2018.


* Based on the results of this study, recommendations for improved management of the KZN shore fishery include:
   (i) follow-up using an independent roving-creel survey;
   (ii) improvement of coastal law enforcement;
   (iii) implementation of a linefish observer programme; and
   (iv) a repeat of the aerial survey every 10 years.


pdf available.


Reference:Mann, B. Q. & J. B. Mann-Lang (2020). Trends in shore-based angling effort determined from aerial surveys: a case study from KwaZulu-Natal, South Africa. African Journal of Marine Science, 42 (3): 269-281.