Control of weeds, predators, and intrusive animals is a critical aspect of successful grow-out fish production in Kenya. Weeds in fish ponds can reduce oxygen levels, interfere with feeding, and provide shelter for predators, thereby impacting fish health and growth rates. Predators and intrusive animals can cause direct losses through predation or indirect losses by damaging pond infrastructure and stressing fish stocks. Effective management ensures optimal pond conditions, maximizes fish survival, and enhances production efficiency for commercial and subsistence fish farmers alike.

5.1 Types of weeds found in ponds

Weeds in fish ponds are aquatic plants that can affect water quality, oxygen availability, and fish movement. In Kenya, pond weeds are categorized based on their growth habits and locations in the water body. Understanding these types helps aquaculture professionals identify and implement appropriate control measures to maintain pond health and productivity.

5.1.1 Submerged plants

Submerged plants grow entirely beneath the water surface, rooted in the pond bottom but with stems and leaves extending underwater. They play a complex role in pond ecosystems but can become problematic when they grow excessively.

Characteristics of submerged plants

• Rooted underwater: These plants anchor themselves in the pond substrate, drawing nutrients directly from sediments.
• Photosynthesis underwater: They carry out photosynthesis below the surface, contributing to oxygen production during daylight.
• Provide habitat: Submerged plants offer shelter for small aquatic organisms, which can be food for fish.
• Can hinder fish movement: Dense growth may obstruct fish swimming and feeding activities, reducing growth rates.
• May contribute to oxygen depletion at night: Respiration by these plants during darkness can reduce dissolved oxygen, stressing fish.

Common submerged weeds in Kenyan ponds

• Hydrilla verticillata: Known for rapid growth, it can form thick mats that clog water flow.
• Ceratophyllum demersum (Hornwort): Thrives in nutrient-rich ponds, competing with fish for oxygen.
• Elodea canadensis: Introduced in some regions, it spreads quickly and can dominate pond bottoms.
• Potamogeton species: Native plants that can become invasive under nutrient-rich conditions.
• Vallisneria spiralis: Often used for habitat but can become excessive if unchecked.

Impact on pond water quality

Submerged plants absorb nutrients, which can reduce algal blooms, but excessive growth increases organic matter upon decay, leading to higher biochemical oxygen demand (BOD). This process can lower dissolved oxygen, especially at night or during decomposition phases, adversely affecting fish health.

Control methods for submerged plants

• Mechanical removal: Hand pulling or using rakes to reduce biomass.
• Biological control: Introducing herbivorous fish such as grass carp that feed on submerged plants.
• Chemical control: Use of approved aquatic herbicides carefully applied to avoid harming fish.
• Water level manipulation: Draining ponds temporarily to expose and dry plants.
• Nutrient management: Reducing nutrient inputs to limit plant overgrowth.

5.1.2 Emergent plants

Emergent plants are rooted underwater but have stems, leaves, and flowers that rise above the water surface. They often colonize pond edges and shallow zones, influencing pond ecology and fish production.

Characteristics of emergent plants

• Rooted in pond bottom: Like submerged plants, they take nutrients from sediments.
• Above-water growth: Leaves and stems extend above the water, allowing gas exchange and photosynthesis.
• Stabilize pond banks: Their roots prevent erosion, which can be beneficial for pond integrity.
• Provide shelter for predators: Dense emergent vegetation can harbor snakes, birds, and other fish predators.
• Can reduce sunlight penetration: Overgrowth shades pond water, limiting photosynthesis by algae and submerged plants.

Common emergent weeds in Kenya

• Typha species (Cattails): Thrive along pond margins, forming dense stands.
• Phragmites (Reeds): Aggressive colonizers of shallow water and wet soils.
• Cyperus papyrus (Papyrus grass): Common in wetlands, can invade ponds with shallow edges.
• Eichhornia crassipes (Water hyacinth) roots along edges: While mainly floating, roots can behave like emergent vegetation near shorelines.
• Schoenoplectus species (Bulrushes): Found in soft sediments, obstructing water flow.

Effects on fish production

Emergent plants can provide refuge for juvenile fish but may also shelter predators, reducing fish survival. Their dense growth can interfere with pond management activities such as feeding, harvesting, and water circulation. Nutrient uptake by emergents can reduce eutrophication but excessive growth leads to organic matter accumulation.

Management of emergent plants

• Physical removal: Cutting or uprooting plants along pond banks.
• Controlled grazing: Using livestock such as goats to clear emergent vegetation.
• Chemical herbicides: Selective herbicides applied carefully to pond margins.
• Buffer zones: Maintaining vegetation-free margins to prevent spread.
• Regular monitoring: Early detection to prevent establishment of dense stands.

5.1.3 Floating plants

Floating plants rest on the water surface and are not rooted in the pond bottom. They can rapidly cover pond surfaces, affecting water quality and fish behavior.

Characteristics of floating plants

• Free-floating: They drift with water currents and wind, not anchored to the substrate.
• Rapid reproduction: Many species reproduce vegetatively, enabling quick coverage.
• Shade water surface: Blocking sunlight, they reduce photosynthesis by submerged plants and algae.
• Consume oxygen at night: Respire continuously, decreasing dissolved oxygen levels, especially at night.
• Interfere with oxygen exchange: Their dense mats reduce gas exchange between water and atmosphere.

Common floating weeds in Kenyan fish ponds

• Lemna species (Duckweed): Small, fast-growing plants that form dense mats.
• Azolla species: Nitrogen-fixing fern that can rapidly cover ponds.
• Salvinia molesta (Giant Salvinia): Aggressive invader of water bodies, difficult to control.
• Eichhornia crassipes (Water hyacinth): One of the most problematic aquatic weeds in Kenya.
• Pistia stratiotes (Water lettuce): Forms thick floating mats, obstructing water flow.

Impact on pond ecosystem

Floating plants reduce light penetration, suppressing beneficial algae and submerged plants, which are vital for oxygen production. Their dense mats can cause oxygen depletion at night, stressing or killing fish. They also obstruct feeding and harvesting operations and can harbor disease vectors.

Control strategies for floating plants

• Manual removal: Skimming or netting floating mats regularly.
• Biological control: Introducing weevil species or fish that consume floating plants.
• Chemical herbicides: Selective aquatic herbicides applied with care.
• Mechanical harvesters: Machines designed to remove floating vegetation at scale.
• Nutrient control: Reducing nutrient runoff to limit plant proliferation.

5.1.4 Algal blooms

Algal blooms are rapid increases in algae populations, often triggered by nutrient enrichment, impacting pond health and fish production.

Causes of algal blooms in fish ponds

• Excess nutrients: High levels of nitrogen and phosphorus from fertilizers, feeds, or runoff stimulate algal growth.
• Warm temperatures: Tropical climates like Kenya’s favour fast algal reproduction.
• Stagnant water: Poor circulation allows algae to accumulate.
• High sunlight: Promotes photosynthesis and bloom formation.
• Imbalanced pond ecosystem: Lack of natural algal predators or competitors.

Types of algae involved in blooms

• Phytoplankton: Microscopic free-floating algae that form green water.
• Blue-green algae (Cyanobacteria): Can produce toxins harmful to fish and humans.
• Filamentous algae: Stringy algae that can form mats on pond surfaces.
• Diatoms: Silica-walled algae common in nutrient-rich waters.
• Green algae: Diverse group contributing to oxygen production but can bloom excessively.

Effects of algal blooms on fish ponds

Algal blooms can initially increase oxygen during the day but cause oxygen depletion at night due to respiration. Some blooms produce toxins causing fish mortality or health issues. Blooms reduce water clarity, interfering with fish feeding and growth. Decomposition of dead algae increases BOD, leading to poor water quality.

Management of algal blooms

• Nutrient reduction: Limiting feed waste and fertilizer runoff.
• Aeration: Mechanical aerators to maintain oxygen levels.
• Algaecides: Chemical treatment applied judiciously to avoid side effects.
• Biomanipulation: Introducing filter-feeding fish to consume algae.
• Water exchange: Partial draining and refilling to dilute nutrient concentrations.

Chapter Summary

This chapter explored the various types of weeds commonly found in fish ponds, including submerged, emergent, and floating plants, as well as the occurrence of algal blooms. It outlined effective methods for controlling these weeds to maintain a healthy aquatic environment conducive to fish growth. The discussion then shifted to fish predators and intrusive animals, highlighting the critical differences between these two groups. Various types of intrusive animals were examined, focusing on their behaviors and how they impact fish production. Finally, the chapter detailed practical approaches for controlling predators on fish farms to protect stock and optimize production outcomes. Together, these topics provide a comprehensive understanding of managing pond ecosystems to support successful grow-out fish production.