Floundering in the mud – can we predict estuarine fish diversity and numbers?
Science Centres: Aquatic Biodiversity and Biosecurity
Yellow-eyed mullet distribution and abundance.
Snapper
Sand goby
Grey mullet
Beach-seine catch of snapper, kahawai, globefish, trevally, garfish, and yellow-eyed mullet.
It would be great if we could preserve or enhance the diversity and numbers of fish in New Zealand’s estuaries and harbours, but to do this we need to know which habitats within estuaries and harbours are important for which fish species. If we know this, then we can better judge the likely effect of human activities, such as erosion, pollution, dredging, fishing, and marina development, on fish communities.
Estuaries and shallow harbours are on the back doorstep of all our large coastal cities. Despite this, we have much to learn about the marine life in them. We are unable to answer many basic questions, including: What species live there? How many are there? What ecological processes influence them? What effect do human activities have on them?
NIWA, through the National Centre for Aquatic Biodiversity & Biosecurity, set out to answer some of these questions in relation to fish as part of a Foundation for Research, Science & Technology funded research project. In February 2001 we carried out a large-scale beach-seine survey of 25 estuaries around the northern North Island to identify large- and medium-scale spatial patterns in the distribution and habitat use of fish. We used fine-mesh beach seines at low tide to sample the fish because this is when they are concentrated in the channels bordering intertidal flats. More than 71 000 small fish and 39 species were caught at 305 sites between Kawhia and Ohiwa Harbour.
Most of the fish we caught were less than 100 mm long, and were either small species or juveniles of larger species. Yellow-eyed mullet, exquisite goby, anchovy, and smelt dominated our catch. The most abundant commercial species were sand flounder, yellow-belly flounder, grey mullet, and snapper.
Yellow-eyed mullet, yellow-belly flounder, and sand flounder were common throughout the region. The two flounders were mostly found in the muddy upper reaches of estuaries, whereas yellow-eyed mullet were common everywhere. Although yellow-eyed mullet has little recreational or commercial importance, its abundance in all estuaries indicates that it is extremely important ecologically. It is undoubtedly a major food source for predatory fish and birds (e.g., shags). Anchovies and smelt were patchily distributed, the latter associated with high freshwater inflows. Some species (sand goby, parore, spotty, and snapper) were much more abundant on the east coast of the North Island than the west coast, and all were associated with seagrass beds. Grey mullet and exquisite goby were caught mainly on the west coast, the latter particularly in Kaipara Harbour.
Our next step was to determine whether the number of any of the fish species was related to the physical or chemical features of the environment. If there was a relationship, then we might be able to use those features to predict the number of fish in places not sampled. We tested a wide range of variables, including estuary or harbour type, distance from the open sea, water temperature, salinity, turbidity, habitat type, harbour characteristics (e.g., area, depth) and catchment characteristics (e.g., rainfall, freshwater runoff).
We found that the number of some fish species is closely correlated with certain variables. For example, sand goby numbers are highest in areas with clear water and seagrass beds, whereas exquisite gobies are found mainly in muddy areas in the upper reaches of the estuary. The two species are rarely found together. Yellow-belly flounders prefer the muddy, turbid, low-salinity upper reaches of estuaries, and avoid seagrass.
These results bode well for our ability to predict estuarine fish numbers. However, we can nearly always improve our predictions if we know which harbour or estuary the fish live in. This indicates that one or more other factors influencing the numbers of fish have not been included in our analyses; this might be because we do not currently have sufficient data or because we do not know enough about the biological requirements of each species to know what variables to use.
Another problem is that some fish species showed little relationship with any of the variables (e.g., yellow-eyed mullet, sand flounder, snapper), so we are unable to predict their numbers accurately. Unfortunately, these species are all important either ecologically or for fisheries. Yellow-eyed mullet are common just about everywhere in estuaries, so it is probably not necessary to be able to precisely predict their numbers, but sand flounder and snapper are much more localised. Snapper were usually found in association with seagrass beds in east coast estuaries, although some seagrass beds had no snapper.
We recently extended our fish sampling in estuaries to depths greater than 10 m by towing a beam-trawl net behind a sampling barge. This showed that juvenile snapper are common in the deep channels and central regions of many estuaries – areas that cannot be sampled by beach seines. Beach seines sample only the fringes of snapper habitat in estuaries, which may explain our inability to predict the number of snapper in our beach-seine samples. Further work is now underway to determine the importance of the shallow and deep parts of estuaries for snapper (and other species).
In early 2003 we carried out another beach-seine survey of six new estuaries in the northern North Island. When these results have been analysed we will compare them with the earlier predictions we made for those same harbours.