Origin of the mass mortality of the flathead grey mullet (Mugil cephalus) in the Tanshui River, northern Taiwan, as indicated by otolith elemental signatures

Reading the biomineralized book of life: expanding otolith biogeochemical research and applications for fisheries and ecosystem-based management

Chemical analysis of calcified structures continues to flourish, as analytical and technological advances enable researchers to tap into trace elements and isotopes taken up in otoliths and other archival tissues at ever greater resolution. Increasingly, these tracers are applied to refine age estimation and interpretation, and to chronicle responses to environmental stressors, linking these to ecological, physiological, and life-history processes. Here, we review emerging approaches and innovative research directions in otolith chemistry, as well as in the chemistry of other archival tissues, outlining their value for fisheries and ecosystem-based management, turning the spotlight on areas where such biomarkers can support decision making. We summarise recent milestones and the challenges that lie ahead to using otoliths and archival tissues as biomarkers, grouped into seven, rapidly expanding and application-oriented research areas that apply chemical analysis in a variety of contexts, namely: (1) supporting fish age estimation; (2) evaluating environmental stress, ecophysiology and individual performance; (3) confirming seafood provenance; (4) resolving connectivity and movement pathways; (5) characterising food webs and trophic interactions; (6) reconstructing reproductive life histories; and (7) tracing stock enhancement efforts. Emerging research directions that apply hard part chemistry to combat seafood fraud, quantify past food webs, as well as to reconcile growth, movement, thermal, metabolic, stress and reproductive life-histories provide opportunities to examine how harvesting and global change impact fish health and fisheries productivity. Ultimately, improved appreciation of the many practical benefits of archival tissue chemistry to fisheries and ecosystem-based management will support their increased implementation into routine monitoring.

Graphical abstract

Using machine learning to understand the implications of meteorological conditions for fish kills

Fish kills, often caused by low levels of dissolved oxygen (DO), involve with complex interactions and dynamics in the environment. In many places the precise cause of massive fish kills remains uncertain due to a lack of continuous water quality monitoring. In this study, we tested if meteorological conditions could act as a proxy for low levels of DO by relating readily available meteorological data to fish kills of grey mullet (Mugil cephalus) using a machine learning technique, the self-organizing map (SOM). Driven by different meteorological patterns, fish kills were classified into summer and non-summer types by the SOM. Summer fish kills were associated with extended periods of lower air pressure and higher temperature, and concentrated storm events 2–3 days before the fish kills. In contrast, non-summer fish kills followed a combination of relatively low air pressure, continuous lower wind speed, and successive storm events 5 days before the fish kills. Our findings suggest that abnormal meteorological conditions can serve as warning signals for managers to avoid fish kills by taking preventative actions. While not replacing water monitoring programs, meteorological data can support fishery management to safeguard the health of the riverine ecosystems.

Variation in habitat use along the freshwater-marine continuum by grey mullet Mugil cephalus at the southern limits of its distribution

In this study, habitat use by Mugil cephalus was investigated in the waters of the west coast of the North Island of New Zealand by analysing microchemical composition of otoliths (laser-ablation inductively coupled plasma mass spectrometry) obtained from individuals from commercial fish stocks and research surveys. Results of this study show that M. cephalus at the southern limits of its distribution display highly flexible migratory behaviour with extensive use of freshwater and brackish habitats, potentially enabling them to maximize foraging opportunities. Mugil cephalus can tolerate a wide range of salinities and can therefore utilize higher productivity areas, such as estuaries and eutrophic riverine lakes. Finally, M. cephalus populations across a range of climates and latitudes appear to differ in the extent to which they utilize freshwater and brackish habitats, possibly with increasing penetration of fresh waters with increasing latitude.