PhD for IAPETUS2 DTP, NERC – The contribution of mesopelagic fish to the functioning of the Southern Ocean Ecosystem in a changing climate – an isotopic study.

Start Date: October 1st 2025

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.     

Methodology 

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 d34S measurements have made large projects feasible. Little if anything is known about the spatial (or vertical) variation of d34S 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.  

Detailed information about the project and schedule can be found at: 

https://iapetus2.ac.uk/studentships/the-contribution-of-mesopelagic-fish-to-the-functioning-of-the-southern-ocean-ecosystem-in-a-changing-climate-an-isotopic-study-2/ 

How to Apply: Please refer to the following website for details on how to apply: 

https://iapetus2.ac.uk/how-to-apply/ AND 

https://www.gla.ac.uk/postgraduate/howtoapplyforapostgraduateresearchdegree/  

Eligibility:

All applicants need to meet NERC’s eligibility criteria to be considered for an IAPETUS2 studentship and these are detailed in the current UKRI studentship terms and conditions: https://www.ukri.org/wp-content/uploads/2022/04/UKRI-021122-TrainingGrantTermsAndConditionsGuidance.pdf#page=11  

Any of our studentships can be part-time if that mode of study is more appropriate for your circumstances. (International students should check the terms of the Tier 4 visa scheme to see if this is possible). 

Applicants may only apply to one IAPETUS2 project in any given year. Where applicants submit more than one application, only the final application submitted before the deadline will be considered.