What are they?
Fish eggs and larvae are the juvenile stages of a fish’s life cycle, they are critically important to the survival and development of fishes. Scientists estimate that there are around 33,600 different species of fish in the world (Vic Lang’at Junior, 2019). Fish eggs are generally round; however they may be ovoid as is the case with the anchovy, or pear shaped for certain gobies (Blaxter, 1969). Once hatched, the larva is usually transparent with some pigment spots of unknown function (Blaxter, 1969). The early stages of muscle development and the notochord (primitive spinal cord) are obvious with minor ossification in the skeleton or development of cartilage (Blaxter, 1969). Fish can release incredible amounts of eggs in one go. For example; ling may release as much as 20 to 60 million eggs (https://shiny.marine.ie/speciesdash/)!
Fish eggs depend on the nutritional content of the yolk to ensure their survival to the larval stage (Murakami et al., 1990). On the other hand, larvae depend on their ability to find food within the water column. Larvae are euryphagous in their early development meaning they will eat anything that is small enough (Hunter, 1980). This ranges from other fish larvae, copepods, phytoplankton, dinoflagellates and naupliar (Hunter, 1980). A major source of mortality of fish eggs and larvae comes from predation (Purcell, 1985). Pelagic fish eggs and larvae have a negligible handling time for predators (McGurk, 1986). Pelagic fish eggs and larvae have a limited sensory system and poorly developed musculature so are easy to capture and consume by predators. However, once fish larvae begin to school, they become more adept at evading predators, and these predators will have to spend longer trying to capture the fish. At this point, a fish develops predator avoidance behaviour (McGurk, 1986). A cystonect siphonophores (a common predator) diet can be made up of as much as 90-100% fish larvae, however, the diets of other gelatinous predators contain much smaller percentages (Purcell, 1985).
Lifespan and the duration at which species are in their different life stages vary dramatically. Climate and life strategy plays an important role in how long a species lives for. Warm-water fish species have egg stages that last from less than 24hours, whereas, cold-water fish species have egg stages longer than two weeks (Purcell, 1985). Climate also has a similar impact on the duration of the larval stage (Purcell, 1985). Orange roughy, which live in the deep ocean, have a lifespan of around 150 years. Meanwhile, the three Eviota species (coral reef fish) studied by Depczynski and Bellwood (2006) had lifespans of less than 100 days!
A lot of fish species always spawn on fixed grounds (Norcross and Shaw, 1984). Fish larvae depend on suitable currents and enough and appropriate food on their way to the nursery area so that they reach it at the right time, condition and size (Norcross and Shaw, 1984). Many temperate fish species spawn in late winter or early spring, which is before most predators become abundant (Purcell, 1985). Coastal species may increase their reproductive potential by having high fecundities, multiple spawning sites or protracted spawning seasons to counteract the mortalities associated with “unpredictable” environments (Norcross and Shaw, 1984). Spawning at night by some fish species may reduce visual predation on the eggs (Purcell, 1985).
Fish eggs and larvae are meroplanktonic as they only spend a portion of their life cycle in the plankton. Fish egg size is species dependent. However, on average they are around 1mm in diameter, the size of grain of sand. Fish larvae size is also species dependent. However, in general fish larvae are microscopic (2-4mm) at the time of hatching (Baensch 2020).
By Éanna Hyland