8.3 Behavior and ecology of marine fishes

Marine fish have diverse feeding strategies, from herbivores munching on algae to carnivores hunting other fish. These strategies shape their roles in ecosystems, influencing food webs and habitat health. Understanding these feeding habits is key to grasping marine ecology.

Fish reproduction is equally varied, with some species broadcasting eggs into the water and others carefully guarding their nests. Social behaviors like schooling and cleaning symbiosis further showcase the complexity of fish life. These behaviors are crucial for survival in the vast ocean.

Feeding Strategies and Trophic Relationships

Feeding strategies of marine fishes

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• Herbivorous fishes consume algae and seagrasses (parrotfishes, surgeonfishes, rabbitfishes)
• Planktivorous fishes feed on zooplankton and phytoplankton (anchovies, herrings, sardines)
• Carnivorous fishes divided into piscivores that prey on other fishes (sharks, tunas, barracudas) and invertebrate feeders that consume crustaceans, mollusks, and other invertebrates (groupers, snappers, goatfishes)
• Omnivorous fishes consume both plant and animal material (grunts, porgies)
• Detritivorous fishes feed on detritus and organic matter (mullets, some gobies)

Ecological roles in marine habitats

• Trophic cascades occur when changes in top predator populations affect lower trophic levels altering ecosystem dynamics
• Keystone species have disproportionately large effects on their ecosystems relative to their abundance and play crucial roles in maintaining ecosystem balance

Reproductive Behaviors and Life Cycles

Reproduction and life cycles

• Spawning strategies include broadcast spawning where eggs and sperm are released into the water column (cod, groupers, snappers) and demersal spawning where eggs are laid on the seafloor or attached to structures (damselfish, clownfish, triggerfish)
• Larval development involves a planktonic larval stage where larvae drift with currents and feed on plankton followed by settlement where larvae transition to the juvenile stage and settle in suitable habitats
• Parental care strategies include mouthbrooding where parents hold fertilized eggs or larvae in their mouths (cardinalfishes, jawfishes) and nest building and guarding where parents construct and defend nests (sticklebacks, gobies, blennies)
• Hermaphroditism occurs in some marine fishes and can be sequential with sex change during their lifetime (protandry: male to female in clownfish and wrasses; protogyny: female to male in parrotfishes and groupers) or simultaneous with possession of both male and female reproductive organs (some seabasses)

Social Behaviors and Interactions

Social behaviors in marine fishes

• Schooling involves the formation of coordinated groups for foraging, migration, and predator avoidance (anchovies, herrings, sardines)
• Shoaling refers to loose aggregations of fish for social reasons, not necessarily coordinated (grunts, snappers)
• Territoriality involves the defense of specific areas for feeding, mating, or shelter (damselfishes, surgeonfishes, butterflyfishes)
• Cleaning symbiosis occurs when cleaner fishes remove parasites and dead skin from other fishes (cleaner wrasses, gobies)
• Predator-prey relationships shape behaviors such as camouflage and cryptic coloration for predator avoidance, schooling and shoaling for safety in numbers, and aposematic coloration with warning colors to deter predators (some butterflyfishes)
• Interspecific competition for resources between different species leads to niche partitioning where resources are divided to minimize competition

Ecological Roles and Human Impacts

Ecological roles in marine habitats

• Coral reefs: herbivorous fishes control algal growth and maintain coral health while predatory fishes control population sizes of lower trophic levels
• Mangroves and seagrass beds serve as nursery habitats for juvenile fishes providing shelter and food resources
• Open ocean: pelagic fishes form the basis of many marine food webs and migratory fishes connect different ecosystems
• Deep sea fishes adapted to high pressure, low light, and scarce food resources contribute to nutrient cycling and energy transfer

Human impacts on fish populations

• Overfishing leads to depletion of fish stocks due to excessive fishing pressure disrupting food webs and ecosystem balance
• Habitat destruction involves the degradation of crucial habitats like coral reefs, mangroves, and seagrass beds resulting in the loss of spawning, nursery, and feeding grounds
• Pollution from industrial waste, agricultural runoff, and plastic debris causes contamination and bioaccumulation of toxins in fish tissues
• Climate change impacts include ocean acidification affecting fish physiology and behavior and rising temperatures altering species distributions and ecosystem dynamics
• Invasive species introduced to new areas can disrupt local ecosystems through competition with native species for resources
• Marine protected areas (MPAs) are designated areas with restricted human activities aimed at conserving fish populations and habitats promoting the recovery of overexploited species and ecosystem resilience