Biology of Finfish and Shellfish (2+1)

4.5.2.1. Open substrate spawners

They simply scatter their eggs in the environment and they do not have specialized reproductive structures. This group is further divided into two categories.

1. Pelagic spawners (or pelagophils)
2. Benthic spawners.

1. Pelagic Spawners

These are known as pelagophils. They spawn in open waters and this strategy is exhibited by many schooling fish like sardines, mackerels, and tunas. In addition to pelagic fishes some of the demersal benthic fishes, also release pelagic eggs. These pelagic eggs are buoyant and are planktonic in nature. Eggs contain oil globule and lot of water content to ensure floatation.

But these pelagic spawners have some disadvantages.

1. The eggs are exposed to variable wild environmental conditions.
2. These eggs may be eaten by pelagic predators.
3. Low survival rate of spawned eggs and larvae.

2. Benthic spawners

These fishes deposit the eggs on the substratum and eggs are adhesive. They release their eggs on known area and they are mass spawners and there is no courtship behaviour and also they do not care for eggs and young ones. They lay eggs in long strings or thick thread. The benthic spawners are broadly classified into 3 categories.

a. Spawners on coarse bottom

These fishes deposit their eggs in stone gravel. Again here there are 2 categories.

i. Pelagophils: the eggs are deposited on the rocks. Young ones or embryos are drift in the surface water. e.g: sturgeons and white fish.

ii. Lithophils: Eggs are deposited on hard substratum but embryos are retained in the bottom. e.g: minnows and perches.

b. Spawners on Plants

Further divided into two categories

i. Phytolithophils: Eggs are deposited on plants, stones and logs of wood (non obligatory). e.g: herrings and minnows.

ii. Phytophils: Eggs are adhesive and are deposited on aquatic plants (obligatory). e.g: common carp.

c. Spawners on sandy bottom (Psammophils)

The eggs are deposited on the sand. e.g: smelts