What characteristics of estuaries make them good places for fish to lay their eggs?

Best Estuaries are among the most biologically productive ecosystems on the planet. More than two thirds of the fish and shellfish we eat spend some part of their lives in estuaries. These ecosystems also provide many other important ecological functions; they act as filters for terrestrial pollutants and provide protection from flooding. & availability of plenty of food provide nutrient for organisms.

Temperature is one of the main elements influencing reproduction of invertebrates and fish. Since estuaries are shallow and semi-enclosed, temperatures can be slightly higher than in open areas, providing the levels required for some species to lay their eggs. Cold temperatures can negatively affect fish in their planktonic stages, slowing down the hatching and growth of the young.
In estuaries, the eelgrass is found just below the low tide line to where the salinity is lower than 14 ppt. It grows best in water temperatures from 10°C to 20°C, reproduces best between 15°C and 20°C, but can tolerate temperatures between -1.5°C and 30°C. Like other sea grasses, it has salt glands on its leaves to remove sodium and chlorine ions from its cells.
Plants and animals common to these areas must have the ability to exist in both salt and fresh water as the tide flow, along with fresh water runoff, create a continuous change in salinity.
Incoming tides produce higher concentrations of salt while falling tides, along with drainage and runoff, result in lower salinity levels. In addition, much of the estuary bottom can be exposed several times a day during low tide.

Redirecting waters for the sake of engineering consequences could be very threatening to local backwaters and the many forms of sealife included.

Another concern is man’s intervention causing the change in salinity levels at various tide stages. Disruption to salinity levels can be disastrous to the delicate life in these areas.