Stable isotope analysis of fish eye lenses: reconstruction of ontogenetic trends in spatial and trophic ecology of elasmobranchs and deep-water teleosts

Studying ontogenetic trends in diet and habitat use of endangered sharks and deep-water teleosts is logistically challenging and expensive, due to the remote and inaccessible nature of the pelagic realm, and the extent of many marine migrations. Chemical analysis of inert, organic, incrementally formed tissues represents a window to retrospectively study whole life-history ecology, however these tissues are rare or absent in many fishes. The fish eye lens is a unique tissue, formed via the sequential deposition of protein-filled fiber cells, which undergo no subsequent remodelling once formed. Despite having great potential to record chemical variations reflecting foraging behaviour, lenses have received relatively little analytical attention. In this thesis I have explored the suitability of fish lenses for recovery of retrospective ontogenetic chemical information, focusing particularly on pre-birth and early juvenile life histories in elasmobranchs. I have confirmed consistent relationships between the body size and lens diameter of four study species (Aphonopus carbo, Coryphaenoides rupestris, Lamna nasus and Squalus acanthias), which allows recovery of a body size-referenced lens samples. Growth relationships reveal that a large proportion of lens tissue in elasmobranchs is deposited pre-birth, opening a previously unrecognised opportunity to study maternal provisioning from tissues of the offspring as adults. I have confirmed that transects of stable isotope compositions across lenses show bilateral symmetry, reflecting the sequential deposition of eye lens tissue. Muscle-lens tissue offsets were examined in S. acanthias and C. rupestris, identifying the potential for species-specific differences, possibly driven by variability in taxon-specific lens protein expression. I have then applied the validated lens sampling protocol to recover cross-generational life history movement and diet ecology information in three study species. Focusing on pre-birth ecology requires some understanding of the isotopic relationship between maternal and embryonic tissues. With access to gravid female spiny dogfish, isotopic spacing between maternal and offspring tissues was investigated. However, the measured mother-embryo isotopic offsets are confounded by migration across isotopic provinces, introducing considerable temporal de-coupling between nutrient assimilation fuelling maternal tissue remodelling and yolk sac provisioning, particularly in species with a long gestation period. For the spiny dogfish Squalus acanthias recovered from the North Sea, lens-derived isotope histories revealed that, whilst the southern and central North Sea represent important foraging areas during mature life history, the northern North Sea is more important during gestation, pupping, and early life history. Similarly, porbeagle (L. nasus) lens chemistry indicates that the mothers of individuals caught in the Celtic Sea region forage over a wide area of the north Atlantic. Attracting mothers from a wide geographical area, this nursery area is likely to be important for the conservation of the endangered northeast Atlantic population, where mitigating discarding is a priority to fisheries managers. The isotopic variability of sequential A. carbo lens samples was also investigated in order to address trends in foraging and location throughout individual life history. Lens data provide further evidence of ontogenetic depth and latitudinal movements, consistent with the species’ ontogenetic migration theory. Obtaining a near whole life history record of information relating to an individual’s trophic and spatial ecology using traditional tagging methods is challenging, if not impossible. The lens therefore represents a valuable chemical repository for high-resolution ecological information that can be analysed retrospectively.