IAP-24-055

The contribution of mesopelagic fish to the functioning of the Southern Ocean Ecosystem in a changing climate – an isotopic study.

Myctophid fish comprise a high biomass in the Southern Ocean (Dornan et al. 2022), which plays a crucial role in food web operation and biogeochemical cycling (Murphy et al. 2007; Saunders et al. 2019; Belcher et al. 2019). However, there is evidence to suggest that much of this high biomass is formed of species that are centred at sub-Antarctic to temperate latitudes, existing only as expatriate sink populations at latitudes south of the Antarctic Polar Front (APF) (Saunders et al. 2017). There appears to be little local reproduction of myctophids in Antarctic waters. Thus, large scale transport and behavioural mechanisms will be crucial in sustaining the high level of myctophid fish biomass in Antarctic waters, which is critical in maintaining local ecosystem function. Little is known about these connectivity mechanisms and pathways and there is a pressing need for new data to better understand how changes in connectivity pathways may impact Antarctic food web and ecosystem function (Murphy et al. 2021). There is increasing evidence that stocks of the keystone species Antarctic krill, which is central in local food webs and sustains many higher predator species in this region, are either declining or redistributing due to broad-scale environmental change (Atkinson et al. 2009, 2019). Such change is likely to result in a change in local food web structure whereby populations of globally important higher predator species (penguins, seals, seabirds) will need to turn increasingly to myctophid fish as an alternative food source as krill become less available in the system (Murphy et al. 2007). However, it remains unclear the extent to which myctophid fish can support higher predator populations under predicted scenarios of krill declines, and what the implications of a such a change in food web structure will be for myctophid fish populations (Saunders et al. 2019). Thus, it is crucial to understand the connectivity mechanisms that control and sustain the influx of myctophid fish biomass south of the APF for effective management of the Southern Ocean ecosystem.

The student will undertake a combination of laboratory-based analyses, principally stable isotope measurements, to understand the connectivity of a selection of myctophids. A model of the spatial variation (=isoscape – St.John Glew and Espinasse et al. 2021) of the C and N stable isotope composition of surface and midwater particulate organic matter (POM), will allow the student to match isotope composition with the location that fish tissues were synthesized. Further, these measurements will be made on eye lenses (and potentially otoliths) – incrementally grown, metabolically inert protein that allows sequential isotope measurements and thus temporal profiles of movement (Quaeck-Davies et al. 2018).

The student will also investigate the efficacy of sulfur isotopes to track connectivity in this system. Sulfur is only just beginning to be utilised to elucidate marine systems since (i) researchers for many years assumed wrongly that non-benthic sulfur would be isotopically uniform and identical to marine sulfate and (ii) recent advances in increasing the throughput and quality of sulfur isotope measurements have made large projects feasible. Little if anything is known about the spatial (or vertical) variation of sulfur isotope values in the Southern Ocean, so a comparison of DI-water-flushed POM samples with myctophids will be used to answer this question.

The student will furthermore learn to identify stomach contents of mesopelagic fish and potentially use population genetics to assist the interpretation of migration patterns derived from stable isotope analysis.

Year 1

Training in the methods needed to answer the posed research questions.

Proof of concept and method that shows how sulfur isotopes (and possibly genetics) can be used to study vertical and horizontal migration patterns in Southern Ocean mesopelagic fish.

Presentation of the PhD project at the British Antarctic Survey student symposium.

Year 2

Diet and migration pattern (horizontal and vertical) analysis of Southern Ocean mesopelagic fish communities using net based estimates and acoustic data as well as isotopic mixing models and/or conventional stomach content data.

Presentation at a national or international symposium.

Writing of manuscripts.

Year 3

Global comparison of DVM/horizontal migration and diet of mesopelagic fish between the Southern Ocean and other regions of the world’s oceans (for which samples exist for example from the Benguela upwelling region or the Irish Sea).

Writing of manuscripts.

Year 3.5

Synthesis of PhD thesis

The IAPETUS2 programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners.
The student will be registered at the University of Glasgow and primarily based at SUERC, and thus join the training programme in the College of Science and Engineering, where they will be provided with skills that are essential to a researcher at all future stages of their career.
They will also spend 5-10% of the project time at British Antarctic Survey in Cambridge and University of Southampton (with additional time if required). At BAS they will be trained in polar ecology, fish identification and subsampling of the fish repository and at Southampton they will be trained in dissection and subsampling of eye lenses for isotope work, and spatial aspects of isotope ecology. Fieldwork is a possibility, but the main aims of the project could be realized using existing fish collections.

Specific training will include:
All aspects of eye lens sclerochronology: extraction and preparation, age and growth analysis
Stable isotope ecology, mixing models, and metrics
Stomach content analysis
Ecophysiology and polar ecology of aquatic organisms
Data analysis and visualization
Team working