Salinity changes directly and indirectly affects the fish composition by many ways. It is not a short talk.

As common man I think -- Salinity  and its level may affect every flora and fauna --so, may/will affect the fish composition.

Development and growth (continuous in fish) are controlled by ‘internal factors’ including CNS, endocrinological and neuroendocrinological systems. Among vertebrates, they also are highly dependent on environmental conditions. Among other factors, many studies have reported an influence of water salinity on fish development and growth. In most species, egg fertilization and incubation, yolk sac resorption, early embryogenesis, swimbladder inflation, larval growth are dependent on salinity. In larger fish, salinity is also a key factor in controlling growth. Do the changes in growth rate, that depend on salinity, result from an action on: (1) standard metabolic rate; (2) food intake; (3) food conversion; and/or (4) hormonal stimulation? Better growth at intermediate salinities (8–20 psu) is very often, but not systematically, correlated to a lower standard metabolic rate. Numerous studies have shown that 20 to >50% of the total fish energy budget are dedicated to osmoregulation. However, recent ones indicate that the osmotic cost is not as high (roughly 10%) as this. Data are also available in terms of food intake and stimulation of food conversion, which are both dependent on the environmental salinity. Temperature and salinity have complex interactions. Many hormones are known to be active in both osmoregulation and growth regulation, e.g. in the control of food intake. All of these factors are reviewed. As often, multiple causality is likely to be at work and the interactive effects of salinity on physiology and behaviour must also be taken into account.

http://www.sciencedirect.com/science/article/pii/S153204560100268X

A re-circulating seawater system employing biological filtration was set up to assess the growth potential of sea bream (Sparus sarba) under the combined effects of salinity (7, 15, 35 p.p.t.) and dietary protein level (30, 35, 40, 45, 50, 55%). The re-circulating system was able to sustain growth of sea bream at high stocking density (3 g 1−1), possibly by minimising stress. Typical stress indices such as serum cortisol, glucose and lactate levels were reduced in sea bream cultured within the recirculating system. Growth rates and protein efficiency ratios of sea bream cultured at 15 p.p.t. were consistently higher than those at other salinities. Growth enhancement at 15 p.p.t. was accompanied by a reduction in oxygen consumption, ammonia excretion rates, liver lipid levels and hepatic glucose-6-phosphatase activity. However, hepatic hexokinase activity was stimulated at 15 p.p.t. Most of these effects were more evident in fish fed high protein levels. These data suggest that growth enhancement at 15 p.p.t. may be explained by (1) reduction of metabolic cost of osmoregulation, and (2) reorganisation of metabolism, which would allow protein sparing in favour of a shift towards preferential utilisation of carbohydrate and lipid.

http://www.sciencedirect.com/science/article/pii/0044848695010033

In my opinion , In general salinity affect the fish's life where we see the difference between the meat of the river Fishو punctures [Fish and Sea Fish  and normally and the then the salinity affect on fin-fish?

Trash fish variety has been found to increase in hypersaline environment.

cbpcservices.com

Not my area just reading what other contribution.

Tilapia zilli in Lake Qaru as water salinity increases progressively, which affects greatly the lake biota, in addition, the exacerbation of eutrophication of the lakeʼs water that caused by the nutrient load from the agricultural drainage water. Information on  water, protein, lipid, ash, and carbohydrate content are available.

http://www.aensiweb.com/old/jasr/jasr/2013/3857-3872.pdf