Seasonal distribution of fish larvae in mangrove-seagrass seascapes of Zanzibar (Tanzania)

Barnabas Tarimo,Matern S P Mtolera,Martin Gullström,Christopher A Muhando,Monika Winder

doi:10.1038/s41598-022-07931-9

Barnabas Tarimo, Matern S P Mtolera + Show 3 more

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https://doi.org/10.1038/s41598-022-07931-9

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Abstract

Fish larvae supply in nearshore vegetated habitats, such as seagrass meadows and mangroves, contributes significantly to sustainable fish stocks. Yet, little information is available on distribution patterns of fish larvae in mangrove and seagrass habitats of the western Indian Ocean. The present study investigated the abundance, diversity and assemblage composition of fish larvae in mangrove creeks, inshore seagrass meadows (located adjacent to mangroves) and nearshore seagrass meadows (located in-between mangroves and coral reefs) in two coastal seascapes of Zanzibar (Tanzania) across seasons for 1 year. The highest mean abundances of fish larvae were recorded in mangrove creeks, while nearshore- and inshore seagrass meadows showed similar mean abundance levels. Generally, fish larvae representing 42 families were identified, with the fourteen most abundant families comprising 83% of all specimens. Fish larvae communities were dominated by specimens of the postflexion growth stage in all habitats, except in mangrove creeks in one of the two seascapes (i.e. Chwaka Bay) from April through June when abundances of the preflexion and very small-sized individuals were exceptionally high. Slightly higher fish larvae abundances were observed in mangroves during the southeast monsoon compared to the northeast monsoon, and there were also differences across months within monsoon periods for all three habitats studied. Assemblage composition of larvae did, however, not vary significantly in time or space. Our findings suggest that mangroves and seagrass meadows are highly linked shallow-water habitats with high retention of fish larvae contributing to similarity in assemblage compositions across shallow coastal seascapes. Conservation and management efforts should prioritize connected shallow-water seascapes for protection of fish larvae and to uphold sustainable coastal fisheries.

Highlights

Fish larvae supply in nearshore vegetated habitats, such as seagrass meadows and mangroves, contributes significantly to sustainable fish stocks
We investigated seasonal distribution patterns of fish larvae in mangrove creeks, inshore seagrass meadows and nearshore seagrass meadows in two tropical coastal seascapes of Zanzibar (Tanzania) in Eastern Africa
Chlorophyll-a levels were substantially higher in mangroves of Chwaka Bay during both the northeast monsoon (NEM) and southeast monsoon (SEM) seasons compared to the levels in mangroves of Fumba and inshore and nearshore seagrass meadows in both sites (Table S1)

Summary

Introduction

Fish larvae supply in nearshore vegetated habitats, such as seagrass meadows and mangroves, contributes significantly to sustainable fish stocks. Numerous fish species use these vegetated coastal habitats as nursery and spawning grounds as the structural complexity provides shelter and avoidance of p redation[9], while calm water in protected environments, such as mangroves, are suitable for fish larvae to settle with often high plankton prey availability[10]. These habitats harbor high abundance of fish larvae[11–13]. Hedberg et al.[41] reported little differences between sites and among habitats (open waters, seagrass meadows and coral reefs) in coastal East Africa, with most fish larvae families occurring in all three habitats without preference. As a result of the lack of static borders and rather high seascape connectivity, shallow-water habitats in tropical coastal seascapes require a broad seascape approach to properly monitor, assess and conserve fish larvae p opulations[42,45]

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