Abstract
There was significant morphological differentiation among samples of Tanganyika sardine Limnothrissa miodon , indicating non‐random association of fish. Individuals within schools appeared to be unrelated as high mtDNA haplotype diversity demonstrated the presence of many maternal lineages in each school. Nevertheless, there was evidence from allozyme analysis for genetic differentiation on a very small geographical scale, possibly even among individual schools, without any clear geographical boundaries among populations. Similar microgeographical differentiation at the allozyme level was found in several marine pelagic species, suggesting a general pattern of random genetic structure in pelagic schooling fish. Such genetic patchiness may arise from biased reproductive success in localized spawning events and long‐term stability of schools, resulting in genetic differentiation among schools. While the present results are preliminary, the far‐reaching implications both for fisheries management and for evolutionary processes in pelagic species warrant further research on microgeographical genetic patterns using more sensitive markers.