Corresponding author: Mohammad Afzal Khan ( khanmafzal@yahoo.com ) Academic editor: Sanja Matić-Skoko
© 2021 Aafaq Nazir, Mohammad Afzal Khan.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Nazir A, Khan MA (2021) Using otoliths for fish stock discrimination: status and challenges. Acta Ichthyologica et Piscatoria 51(2): 199-218. https://doi.org/10.3897/aiep.51.64166
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Otoliths are calcified structures and the information contained within their chemistry or shape can be used to infer life history events, migration patterns, and stock structure of a fish population. Understanding how otolith chemistry is affected by temperature, salinity, interactive effects of abiotic factors, ontogeny, physiology, etc. is essential for the reconstruction of the environment that affected the fish. Otolith shape is also affected by environmental conditions in addition to the genotype. The applications of otolith chemistry and shape for stock discrimination have increased in recent years because of the advancements in analytical methods and the related software. The stock identification methods sometimes provide variable results but if we use complementary approach the information generated could be more reliable which can be used to prepare effective management and conservation strategies. It appears warranted to generate more information on the factors influencing otolith chemistry and shape especially when two or more factors exert synergetic influence. Therefore, the objectives of this review paper were to provide comprehensive information on various factors influencing the otolith chemistry and shape, and the utility of otolith chemistry and shape for fish stock discrimination with an emphasis towards the research areas needing additional studies.
element, fisheries management, otolith microchemistry, otolith shape, salinity, stock identification, temperature
Otoliths are calcified biominerals that are mainly composed of calcium carbonate polymorph aragonite and a minor quantity of biomolecules such as proteoglycans and glycoproteins (
Otolith chemistry reflects the permanent record of physical and chemical qualities of the ambient environment (
Several studies have reported that the otolith chemistry is influenced by environmental factors (e.g., water chemistry, temperature, salinity, and their interactive effects) and physiological processes (e.g., growth, metabolism, and reproductive stage) (
The shape of otoliths is species-specific and shows less variation in growth comparative to somatic growth (
Several studies have used otolith chemistry (
Several review papers based on otolith chemistry have been published both at the global and regional level (
Otolith chemistry reflects the habitat conditions at different life-history stages of a fish species (
Temperature variations at both spatial and temporal scales influence otolith chemistry because it affects precipitation reactions of trace elements.
Several studies have reported the effects of temperature on otolith chemistry, but a full understanding of the relation is lacking because it is complicated by other factors that affect otolith chemistry (such as salinity, diet, species-specific physiology, etc.) and many empirical studies do not replicate the range of conditions that the species of interest experience naturally (
During the assessment of the literature, we found some specific study designs that were particularly good to understand the factors influencing the otolith chemistry, for example,
Salinity changes occur due to precipitation and evaporation processes within water bodies, leading to a stratification of the water column which often delimits boundaries between different water bodies (
Several studies have reported strong positive and negative effects of salinity on otolith chemistry (
The water chemistry, temperature, and salinity are the three key environmental factors influencing the otolith chemistry and these variables are usually not independent of each other (
Laboratory-based validation experiments have been performed to analyse the accretion of elements on the otoliths for a number of species, but a limited number of studies have examined the interactive effect of temperature and salinity (
Several studies have reported that the otoliths vary considerably in the crystal structure and the differences in elemental composition were found among CaCO3 polymorphs (
The growth rate of a fish species is known to influence the elemental incorporation in otolith (
The elemental signatures in otoliths are accreted usually from water but the effect of diet is also significant for few elements such as Sr and Ba (
Otolith chemistry can vary with life history stages (growth from larval to juvenile and juvenile to adult stages) and metamorphosis (
Stress can influence otolith chemistry but the trend and mechanism driving such results are still not clearly known (
The reproductive conditions can influence the ion transport, flux of elements into the blood plasma, and consequently the availability of elements for uptake into otolith (
The exogenous and endogenous factors determine the shape of otoliths and these variables may change between populations thereby resulting in stock-specific characteristics of otoliths (
Several studies have reported abnormal otoliths which have different size, shape, and density as compared to normal otoliths in a number of freshwater and marine fishes (
Diet is also known to influence otolith shape because the composition of the diet may affect saccular endolymph proteins which play an essential role in otolith biomineralization (
Traditionally, the artificial tagging of fish and telemetry are applied to study the life history events (
The utility of otolith chemistry for stock discrimination has risen effectively in the past two decades. The otolith chemistry and its utility in stock discrimination have appeared in nearly 1500 peer-reviewed papers from 2000 to 2019 (Web of ScienceTM, search on 17 September 2020; search term: otolith chemistry OR otolith microchemistry OR otolith elemental composition (black bar), and otolith chemistry OR otolith microchemistry OR otolith elemental composition AND stock discrimination OR stock delineation OR stock deciphering (grey bar)). Approximately 39% of these papers were published in the past five years and the number of papers based on this parameter is increasing with time (Fig.
Number of publications per year featuring otolith chemistry (Web of Science, search on 17 September 2020, search term: otolith chemistry OR otolith microchemistry OR otolith elemental composition (black bar), and otolith chemistry OR otolith microchemistry OR otolith elemental composition AND stock discrimination OR stock delineation OR stock deciphering (grey bar)). Lines indicate the trends and results were not cross-checked.
The majority of the stock discrimination studies using otolith chemistry have not considered the effects of multiple environmental and biological factors that govern the incorporation of elements in otoliths. Therefore, future studies at spatial and temporal scales to disentangle the relative influence of these factors should be undertaken to strengthen our understanding of otolith chemistry and its field applications. Moreover, studies based on otolith chemistry (about 83%) have not described the applications of otolith chemistry for fisheries management (
Otolith shape variation has become widely used for stock discrimination (
Number of publications per year featuring otolith shape (Web of Science, search on 17 September 2020, search term: otolith shape OR otolith morphometry (black bar), and otolith shape OR otolith morphometry AND stock discrimination OR stock delineation OR stock deciphering (grey bar)). Lines indicate the trends and results were not cross-checked.
Otolith shape analysis has been used for stock discrimination in a number of fish species. For example,
During the review of literature, the interest in population discrimination was much lower in freshwater ecosystems where exploitation is less in scale and populations are often well spatially-separated compared to marine ecosystems. In general, the otolith shape method itself is not comparably less used but there are simply fewer analyses of population discrimination in freshwater ecosystems. The utility of otolith shape for stock discrimination has shown an increasing trend because it can be used as a complementary technique that certainly can improve our understanding of the stock structure (
In the past two decades, the number of publications per year based on otolith chemistry and otolith shape is showing an increasing trend (Web of ScienceTM, search on 17 September 2020, search term: otolith chemistry OR otolith microchemistry OR otolith elemental composition (black bar), otolith shape OR otolith morphometry (grey bar), and otolith chemistry OR otolith microchemistry OR otolith elemental composition AND otolith shape OR otolith morphometry (light grey bar)) (Fig.
Number of publications per year featuring otolith chemistry and otolith shape (Web of Science, search on 17 September 2020, search term: otolith chemistry OR otolith microchemistry OR otolith elemental composition (black bar), otolith shape OR otolith morphometry (grey bar), and otolith chemistry OR otolith microchemistry OR otolith elemental composition AND otolith shape OR otolith morphometry (light grey bar)). Lines indicate the trends and results were not cross-checked.
The choice of stock discrimination methods is important because each method has its particular associated challenges such as ecological interpretation, spatio-temporal variations, discriminatory power, and related expenses (
Otolith chemistry in association with growth increments can be used to document the environment information in which fish species live and habitat changes throughout ontogeny (
In recent years, the information contained within otolith chemistry and shape has been commonly used for fish stock discrimination and to interlink management strategies and biological processes. However, the otolith chemistry and shape are influenced by several exogenous and endogenous factors. Several studies have reported that temperature and salinity are important environmental factors influencing otolith chemistry but these studies usually have not utilized a broad range of temperature and salinity typically the fish experience in nature. Comprehensive in situ validation experiments across a complete reproductive cycle should be undertaken to fully understand the relation between these factors and the otolith chemistry. The biokinetic and bioenergetics models can be used to evaluate a suite of environmental and biological factors influencing the uptake of elements in otoliths. The relation between calcium polymorphs and the rate of elemental uptake needs further assessment. To fully utilize otolith chemistry for stock discrimination at both spatial and temporal scales, it is recommended to build a multiyear database of otolith chemistry and water chemistry signatures. The complete understanding of local environmental conditions and genetic factors influencing otolith shape requires further investigations. The introduction of exotic fish species having distinct evolutionary histories can provide an effective method to analyse the influence of environment and genetics on otolith shape. A complementary approach is desirable because of the limitations and assumptions associated with any particular method and as far as proper fisheries management is concerned. Further, a collaboration between different stakeholders should be mediated by fisheries managers about the utility of otolith chemistry and shape to bridge the gap between research and ecosystem-based fisheries management.
The authors are grateful to the Chairperson, Department of Zoology, Aligarh Muslim University, Aligarh, India for providing necessary facilities.