Stream degradation and alteration resulting from human activity is a huge threat to the sustenance of O‘opu populations. As more streams in Hawaii are affected by human alterations, the amount of habitat suitable for O‘opu declines. The amphidromous life cycle of native Hawaiian stream gobioids makes them extremely vulnerable to stream degradation because without viable connections between the stream and the ocean, these species cannot complete their life cycle.
Channel modification:
Channel modification in Hawaii began in the 1930’s and was primarily a result of bridge construction. This type of modification included the removal of riparian vegetation, realignment of channels, and reinforcement of altered banks. Flood control projects that occurred subsequently resulted in many streams having artificially straightened, concrete-lined channels. This type of habitat is a sharp contrast to the natural stream environment, which is heavily vegetated, steep, and strewn with boulders. These types of concrete-lined flat bottom channels cause water temperatures to rise excessively throughout the day, easily exceeding limits tolerated by native stream animals on a daily basis. Also, the homogeneity of the environment provides no hiding-places for stream animals, organisms that have adapted to the rocky, precipitous freshet-flow nature of Hawaiian streams. Because of these reasons, concrete-lined channels are basically not suitable habitat for native stream species.
Dewatering:
During the past century, as sugar became Hawaii’s primary form of agriculture, many streams were totally diverted to supply water to cane fields. Extensive irrigation systems were constructed to transport water from the windward sides of islands to the dry leeward sides that supported crops such as sugar cane, pineapple, and macadamia nut. Many of these diversions draw 100% of the median stream flow into channels and ditches for transport to agricultural fields. This results in the elimination of stream flow below diversions except during major precipitation events, when rising waters are able to breech diversions. This type of diversion is extremely detrimental to O‘opu (as well as native shrimps and snails) that require access to the ocean to complete their amphidromous life cycle.
Introduction of non-native species:
Introductions of alien aquatic species can have several types of negative impacts on native stream organisms. Alien generalist species are typically better adapted to degraded habitats than native species. When they become established in these habitats, they can cause further reduction in native populations both directly and indirectly, through competition, predation, and introduction of parasites and diseases. For example, introduced catfish compete with stream gobies for habitat and can even alter the stream environment to make it more suitable for themselves by digging burrows in stream banks, causing additional erosion and siltation. Small-mouth bass are predators that are known to prey on native fish and invertebrate species. Non-native predators such as poeciliids (“mosquito fish”) may also prey on goby post-larvae as they attempt to migrate upstream or settle in the stream. Rainbow trout were found to prey on native species of insects and shrimps. Poeciliids introduced by the state of Hawaii for mosquito control and subsequently as aquarium releases brought with them a range of parasites that have been transferred to O‘opu.
Two different species of catfish. The top one is a Long-fin armored catfish, while the bottom one is known as a Suckermouthed catfish.
Photo by Yamamoto and Tagawa
Common Mosquitofish found all over the world. The top one is a female, while the bottom one is a male.
Photo by Yamamoto and Tagawa
Tahitian prawn.
Photo by Yamamoto and Tagawa