The European sardine, Sardina pilchardus, and the Pacific and southern African sardine, Sardinops sagax, are the dominant small pelagic fishes of the Canary, California, Humboldt and Benguela current systems (Parrish et al., 1989). Their abundance fluctuates over decades in ways tied to ocean climate (Checkley et al., 2017), and managing them requires a view on how many self-sustaining units the fishery is drawing from. Along an upwelling coast that question is hard to settle. Fish school and migrate alongshore, spawning grounds shift with temperature, and management boundaries usually follow national borders or statistical areas and not biology.

No single method answers it. Current practice in stock identification is to combine markers that respond on different time scales: genetics for reproductive isolation over generations, and phenotypic markers such as morphometrics, parasites, otolith chemistry and otolith shape for separation within a lifetime (Begg and Waldman, 1999; Cadrin et al., 2014). Small pelagic fishes with large populations and high gene flow often show little genetic differentiation even where groups remain apart for most of their lives, and it is here that phenotypic markers earn their place.

Otolith shape is the cheapest of them. Otoliths are already collected for ageing in most monitoring programmes, and a calibrated photograph is all that is needed. The method was established by Campana and Casselman (1993) and has been applied to clupeoids, including herring (Burke et al., 2008) and the European sardine (Jemaa et al., 2015). This article proposes a stepwise procedure for a sardine study along an upwelling coast. It describes method and reasoning only; no samples were analysed.

The sagitta is the largest of the three pairs of otoliths in most teleosts and forms part of the organ of hearing and balance. It grows by daily accretion of calcium carbonate on a protein matrix and, unlike bone, is not resorbed, so its form is the cumulative outcome of the growth of the fish (Campana, 1999). Shape is species-specific enough for fish to be identified from their otoliths alone. Within a species it varies with ancestry and with environment, chiefly through the effect of temperature and food on growth rate.

The relative weight of the two influences has been examined directly. In known-age Atlantic cod reared under controlled conditions, Cardinale et al. (2004) found that both stock origin and rearing environment altered otolith shape. Working with a tropical fish introduced to new waters, Vignon and Morat (2010) concluded that environment and genetics act on different parts of the outline. The implication for an upwelling coast is direct. Persistent alongshore gradients in temperature and productivity will tend to produce differences in otolith shape between fish that spend their lives in different sectors, whether or not they are genetically distinct. Shape therefore identifies phenotypic stocks: groups with different life histories, which is the level at which most assessment models operate.

The procedure has eight steps, shown in Figure 1. They are described in order below. The choice of shape descriptor in the fifth step is the one most debated in the literature, and the options are compared in Table 1.

Three features of upwelling systems call for particular care. The first is that the environmental gradient is itself variable. The intensity of upwelling changes between years and decades, and a baseline built in a cool period may not hold in a warm one. Baselines should be renewed regularly and matched by year class to the fish being assigned. The second is alongshore movement: sardine may be spawned in one sector and recruit to the fishery in another, so the outline of an adult integrates conditions from several places. Restricting the analysis to a narrow age range reduces the ambiguity. The third is that sardine and other small pelagics expand and contract their range with abundance, so that boundaries evident at low biomass may disappear at high biomass.

This procedure is proposed and has not been validated here. Its steps follow published methods, but whether they yield reliable discrimination for sardine on a given coast is an empirical matter. A validation would need baseline samples of spawning fish from each candidate area in at least three years, classification rules built in one year and tested on the others, and a comparison of the shape result with an independent marker on the same individuals. It would also need a measure of imaging repeatability between operators.

Some limits are inherent. Shape is a phenotypic marker and cannot distinguish a plastic response from an inherited one. Classification rates fall as the number of candidate areas rises, and with small otoliths the measurement error is not negligible relative to the differences sought. The method is best regarded as a screening tool that indicates where more expensive markers should be applied.

Otolith outlines can be obtained at little cost from material that sardine monitoring programmes already collect, and the analytical tools are freely available. The eight-step procedure proposed here puts the emphasis where shape studies most often go wrong: in the sampling design, the control of size and age effects, and the honest estimation of classification error. Applied in that way, and read together with other evidence, otolith shape is a reasonable first test of the stock boundaries assumed along an upwelling coast. Its performance for any particular sardine population has still to be demonstrated.

To be declared by the author(s) before publication.

To be declared by the author(s) before publication.