Seasonal fish larvae abundance and composition in seagrass habitats of coastal East Africa – Scientific Reports

Abstract

Seagrass habitats play a major role in fisheries productivity through nursery functions and feeding grounds for diverse fish species. However, little is known about the seasonal distribution of fish larvae at large spatial scales in coastal East Africa. We investigated drivers of the seasonal fish larvae abundance and composition in seagrass habitats in Kenya and Tanzania. We found a high diversity of fish larvae (54 families) inhabiting seagrass habitats that differed between sites and seasons. Fish larvae abundance were highest in Kenya, particularly during the northeast monsoon season. Overall, total larval abundances per site were low, reaching less than 190 individuals/100 m³ in Kenya and less than 40 individuals/100 m³ in Tanzania, likely related to the low productivity and strong hydrodynamic processes in this region. Our data suggests that most of the fish spawn year-round in these tropical waters as we did not find strong seasonal patterns. All sites had a high relative abundance of larvae from demersal spawning fishes, indicating that many fish species move to coastal sites for spawning. Primary productivity and dissolved oxygen, driven by hydrodynamics conditions are positively related to fish larvae productivity both in Kenya and Tanzania. These findings indicate that the occurrence of both resident and transient fish larvae in seagrass meadows is driven by strong hydrodynamic and tidal processes that transport fish larvae across adjacent habitats.

Introduction

Seagrass meadows are among the dominant shallow-water habitats in tropical coastal areas, providing numerous essential ecosystem functions, including the usage as nursery and feeding grounds for plentiful fish species. The structural complexity of seagrass habitats is expected to provide shelter, food and protection from predation for fish larvae and juveniles. The habitat complexity of seagrass meadows in combination with the relatively calm waters of nearshore areas is expected to support adult spawning and fish larval retention, although some larval retention within habitats may also be affected by other factors such as spawning behaviour. Because fish mortality during early life stages is a critical production constraint and a critical factor in ensuring the sustainability of fish stocks, information on the distribution of fish larvae in coastal areas is necessary to provide an essential understanding of fish community biology and seasonal productivity patterns. Yet, little is known about factors influencing the distribution of early life stages of fish, particularly in the coastal habitats of East Africa in the Western Indian Ocean (WIO), a region where fish are an essential source of food and income. Seasonal variability of fish larvae community composition and abundance across shallow-water coastal habitats mainly focused on local area comparisons, covering a relatively narrow spatial scale, which limits the understanding of how environmental conditions and ecological characteristics occurring at a larger scale (tens to hundreds of kilometres) influence the distribution of fish larvae.

Sustainable Development Goals (SDGs) Emphasis

• SDG 14: Life Below Water – This study directly contributes to understanding marine biodiversity, particularly focusing on seagrass habitats which are crucial for marine life.
• SDG 13: Climate Action – By understanding how environmental factors affect fish larvae distribution, this research can inform climate adaptation strategies for marine ecosystems.
• SDG 2: Zero Hunger – Fish are an essential source of food and income in East Africa; thus, understanding fish larvae dynamics can help sustain fisheries and food security.

Materials and Methods

Study Area

The research was carried out in seagrass meadows in four areas in coastal East Africa, including two areas in Kenya (Watamu and Diani), one area in the Tanzanian coastal mainland (Tanga) and one area on Zanzibar Island (Zanzibar). All areas are utilized by artisanal fishers for food and income. The sampling area of Watamu is a shallow lagoon with an average depth of 5 m at high tide. Tidal flushing characterizes this area, while almost the entire area dries out during low tide, except for a few deeper channels. Patch reefs fringe the area on the ocean side where they break the strong waves. Diani is a shallow-water coastal area with an average depth of 3.9 m at high tide. As in Watamu, much of the area in Diani dries out at low tide, leaving water only in a few deeper channels. The offshore currents flow mainly northward with rates of up to 4 knots during the SEM and 3 knots during the NEM, but within the Watamu areas, the currents are weak (< 0.8 knots). The Tanga site is located in a shallow-water area with an average depth of 3.5 m at high tide. The speed of water currents in this area is about 1.9 knots and a small portion of the area is exposed during low tide. The sampling area at Zanzibar Island included the semi-enclosed shallow Chwaka Bay, with a total area of 50 km² at spring high tide and an average depth of 3.2 m at high tide, and Fumba, which is a non-estuarine open coastal area with an average depth of 10 m at high tide and a large exposed area at low tide. In addition to seagrass habitats, the sampling sites of Zanzibar and Tanga comprised fringing mangroves, patches of coral reef and a variety of macroalgae.

Sampling and Sample Analysis

Sampling was conducted for a minimum of three months per season at each site, repeated over two sessions (2 season cycles), except for Zanzibar, where sampling occurred throughout all 12 calendar months. Sampling took place at the sites of Watamu, Diani and Tanga from June 2019 to January 2021, and at the Zanzibar sites from January to December 2018. The sampling frequency was designed to factor in seasonality, i.e., SEM (April–October) and NEM (November–March) seasons. All sites were sampled during the daytime and at high tide (between 06:30 h and 15:00 h). Each site had two different subsites, which were selected based on accessibility and representativeness in the presence of seagrass meadows. Fish larvae were sampled using a plankton net with a removable cod end (mesh size 500 µm), a mouth diameter of 0.5 m and a length of 2.5 m, fixed with an oceanic® flowmeter in the mouth frame to estimate the filtered volume of water. The plankton net was towed horizontally (at an average depth of 1 m) for 15 min behind a motorized boat (at a speed of around 1–1.5 knots, equivalent to 2–3 km per hour) and replicated twice at each station. The net was kept at about 8 m from the rear of the boat to avoid effects of boat wake during sampling. The sampling path was random to maximize larvae capture. The fish larvae specimens were fixed with a 75% ethanol solution until further analysis. During each sampling event, in

Sustainable Development Goals (SDGs) Analysis

1. Which SDGs are addressed or connected to the issues highlighted in the article?

• SDG 14: Life Below Water
• SDG 13: Climate Action
• SDG 2: Zero Hunger
• SDG 15: Life on Land

2. What specific targets under those SDGs can be identified based on the article’s content?

• SDG 14: Life Below Water
  • Target 14.2: Sustainably manage and protect marine and coastal ecosystems to avoid significant adverse impacts.
  • Target 14.4: Effectively regulate harvesting and end overfishing, illegal, unreported, and unregulated fishing.
  • Target 14.5: Conserve at least 10% of coastal and marine areas.
• SDG 13: Climate Action
  • Target 13.1: Strengthen resilience and adaptive capacity to climate-related hazards and natural disasters.
• SDG 2: Zero Hunger
  • Target 2.3: Double the agricultural productivity and incomes of small-scale food producers.
• SDG 15: Life on Land
  • Target 15.1: Ensure the conservation, restoration, and sustainable use of terrestrial and inland freshwater ecosystems.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

• Indicators for SDG 14:
  • Proportion of fish stocks within biologically sustainable levels.
  • Coverage of protected areas in relation to marine areas.
  • Change in fish larvae abundance and community composition in seagrass habitats.
• Indicators for SDG 13:
  • Number of countries with strategies for enhancing adaptive capacity to climate change.
  • Changes in environmental variables such as sea surface temperature, salinity, and dissolved oxygen.
• Indicators for SDG 2:
  • Productivity levels of small-scale fisheries in terms of fish larvae abundance.
• Indicators for SDG 15:
  • Proportion of important sites for terrestrial and freshwater biodiversity that are covered by protected areas.

4. Findings from Analyzing the Article

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Fuente: nature.com

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