The biology and ecology of regionally endothermic fishes in Irish waters
Citation:
Dolton, Haley Rose, The biology and ecology of regionally endothermic fishes in Irish waters, Trinity College Dublin, School of Natural Sciences, Zoology, 2023Download Item:
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Abstract:
The waters surrounding Ireland are very productive due to cold water upwellings. This has led to Irish waters supporting many livelihoods such as those in tourism and fisheries, with some specialising in catching and observing regionally endothermic fishes. Endothermic fishes possess specialised anatomical structures such as centralised red muscle and rete mirabile, which generate and maintain body heat for long periods of time in certain tissues. These endothermic traits are thought to be advantageous for visual perception of prey, increased cruising speed and aiding in the recovery of metabolites after exercise. Most regionally endothermic fishes are rarely encountered due to their predominately pelagic lifestyle. However, anatomical and biologging studies are revealing more about the ecology and biology of these enigmatic species. In chapter 2, I investigated how a regionally endothermic fish, the Atlantic bluefin tuna (ABFT), behaves after a stressful catch-and-release (C&R) event. This highly tuned regional endotherm has the capacity to produce and retain body heat to such an extent that they are capable of overheating and dying during overexertion. Consequently, investigating how they behave after a stressful event is important to understand their behaviour and physiology, with the future aim of improving fishing practices of this ecologically and commercially important species. Catch and release fishing is thought to reduce mortality and stress of captured fish when compared to other fishery interactions. However, the fine-scale behavioural effects of C&R fishing of ABFT are unknown. To address this, I made use of an established C&R fishery and attached biologgers to assess how a regionally endothermic fish behaves after a C&R event. I found that ABFT make initial powered descents and have an elevated tailbeat frequency for approximately 5 to 10 hours post release potentially indicating recovery behaviour after an anaerobic event. I also found changes in the behaviour of fish that were handled differently during the capture event, with one fish suffering mortality. This could indicate that handling techniques of regionally endothermic fish can affect their chances of survival post-release and should be investigated further. Another historically important fish species in Irish waters, the basking shark Cetorhinus maximus, is currently found in large numbers off the west coastline of Ireland and although described as ectothermic in the literature (alongside 99.9% of all other fishes), it behaves in a similar way to regionally endothermic fish. For example, they have an elevated cruising speed and make fast oceanic migrations, indicating that basking sharks might not be full ectotherms. In addition to behaving like a regional endotherm, basking sharks are also found within the same order as regionally endothermic sharks, Lamniformes. In Chapter 3 I investigate basking shark anatomy through dissections of stranded carcasses, and physiology through biologging to test the assumption the species is ectothermic. I found basking sharks have anatomical traits shared with regionally endothermic sharks such as red muscle found closer to the vertebrae at the trunk and a high percentage of compact tissue of the heart (47%). Deploying internal temperature probes into shark species normally involves capture and restraining alongside a boat to insert a temperature logger. However, this is not possible for a planktivorous shark measuring up to 12 m in total length. Consequently, I co-designed and successfully implemented a new biologging technique of inserting and retrieving an internal temperature probe from a free-swimming basking shark. Data from these temperature loggers revealed that basking sharks are able to consistently elevate their body temperature by 1.0 to 1.5?C above ambient, and along with anatomical results from this chapter, demonstrates that basking sharks should now be classed as regionally endothermic. This finding was validated using estimations of heat transfer coefficients (HTC) and whole-body thermal conductance (or k coefficient) for a model ectothermic basking shark and similar-sized and fully ectothermic whale sharks Rhincodon typus. I showed modelled ectothermic shark body temperature quickly converged towards ambient, whereas body temperature data collected from basking sharks remained elevated above ambient. In Chapter 4 I estimated the HTC of large regionally endothermic basking sharks and provided k estimates for the largest regionally endothermic fish species to date. I found basking sharks have a higher k value than predicted for their mass, with their k value increasing by approximately 100 times after a period of high mechanical effort (a triple breaching event). This is possibly explained by their filter-feeding life history increasing bulk flow rate of water over the gills, which in turn, may facilitate heat exchange at the gills. Despite a higher k value than predicted for their body mass, basking sharks are still able to maintain their body temperature above ambient and when increases in mechanical effort were recorded, white muscle temperature rapidly increased. Results from Chapter 3 and 4 questioned when regional endothermy evolved within Lamniformes and which life histories it facilitated as it was a trait thought to be confined to apex predatory sharks. For example, the largest living shark species to have lived, the megatooth shark Otodus megalodon, is assumed to be regionally endothermic having reached that size partly due to its thermophysiology. In contrast, filter-feeding sharks were thought to have only reached their large size due to either their feeding ecology or thermoregulatory strategy. Regional endothermy and filter-feeding were never thought to have co-existed in the same Lamniformes species. However, finding the basking shark is a regional endotherm challenges our understanding of how many times this trait might have evolved in Lamniformes and when it may have evolved. In chapter 5 I explored the presence of anatomical structures associated with regional endothermy in a more basal Lamniform, the smalltooth sand tiger shark Odontaspis ferox. I dissected carcasses, finding red muscle closer to the vertebrae and a high percentage of compact myocardium of the heart (48%). This suggests that perhaps this species is also regionally endothermic, supporting a one origin theory of regional endothermy in shark species and indicating further investigation into the anatomy and physiology of Lamniformes should be conducted to assess the prevalence of this trat within this largely assumed ectothermic order. In summary, I have addressed how hard-to-study regionally endothermic fishes behave in their natural environment and challenged long-held assumptions regarding the anatomy and physiology of Lamniformes. These findings greatly enhance our understanding and knowledge of regional endothermy in fishes and also the evolutionary origin and purpose of this trait in shark species. Only by understanding more about a species are we able to effectively conserve them in an ever-changing environment.
Sponsor
Grant Number
Irish Research Council
Fisheries Society of the British Isles
Description:
APPROVED
Author: Dolton, Haley Rose
Advisor:
Payne, NicholasPublisher:
Trinity College Dublin. School of Natural Sciences. Discipline of ZoologyType of material:
ThesisAvailability:
Full text availableKeywords:
Shark, Regional endothermy, Ectotherm, Basking shark, Lamnidae, HTC, Atlantic Bluefin TunaLicences: