大蠔河是本地獨一無二的地點，是少數的中等規模自然溪流，從高地一直延伸到低地河口。
據報導，大蠔河擁有本港最多的淡水和鹹淡水魚類品種。溪流中共記錄47種，亦包括一些新記錄的香港魚類。
根據記錄，大蠔河是香港淡水魚多樣性最高的一條河，當中包括一些新記錄的香港魚類。其中令人最感興趣的是洄游性魚類，例如高翅舌織魚（香魚），在本港它只在大蠔河能找到。香魚與鮭魚一樣，出生在淡水中，之後遷移到海中成長，最後返回原來的淡水出生地繁殖下代。但是香魚十分稀有， 繼1985年被首次發現後，到1993年才再次捕獲到雌性個體。在大蠔河南端靠近河口的位置，有一些紅樹林和小片海草（貝克喜鹽草）。主要的紅樹品種是桐花樹和秋茄，還可以找到一定規模的本地稀有物種，例如木欖和海草床。它們為各種生活在潮間帶的海洋無脊椎動物和魚類提供庇護所和食物，而紅樹林也是魚類重要的育苗區。

The Tai Ho Stream is unique in the local context. It is one of the few remaining medium-sized natural streams stretching from uplands to the lowland estuary. The Tai Ho Stream was reported as the stream supporting the greatest diversity of fresh water and brackish-water fish in the territory. A total of 47 species, including some newly recorded fishes for Hong Kong, was recorded in this stream.
Of particular interest of the fish fauna is a migratory fish, called Ayu (Plecoglossus altivelis). Tai Ho Stream was reported as the only known location for the species in the territory. Ayu, which is related to the salmon, is born in freshwater and migrates to the sea before returning the freshwater to breed. However, the fish is rarely sighted. The first sighting of this fish was made in 1985 and a female individual was captured again in 1993.
Close to the stream mouth, at the southern end of the Tai Ho Wan, there are some mangroves and small patches of seagrass (Halophila beccarii). The mangroves are dominated by Aegiceras corniculatum and Kandelia candel with a good sized stand of locally uncommon species, Bruguiera gymnorrhiza Mangroves and seagrass beds, in general, provide shelter and food for a variety of intertidal or marine invertebrates and fish. They are also important nursery areas for fish.