Loch Tarbert deeply bisects the south Hebridean island of Jura. It is a relatively small and shallow fiordic loch, some 11 km in length and 43 m at its deepest point. The loch is divided into three basins by the shallow narrows of Cumhann Mor and Cumhann Beag, the latter a very long and tortuous channel. The outer basin, subject to Atlantic swells and exposured to wave action, contrasts with the extremely sheltered and brackish inner basin. There are only weak tidal currents throughout much of the loch, but much stronger currents run through each of the narrows. Despite its proximity to mainland Scotland, and in contrast to most other Scottish lochs, Loch Tarbert is almost completely undeveloped. Only two houses lie beside its shores and the surrounding mountainous land is almost entirely given over to Jura's extensive population of deer. At present there is little mariculture or fishing activity within the loch. The loch was previously surveyed by Smith (1982) and Hiscock (1983) for the Nature Conservancy Council. The present survey, in 1990, complements these earlier surveys and contributes to the MNCR Survey of Scottish sealochs, which is being undertaken jointly by the MNCR and the University Marine Biological Station, Millport. The results from all three surveys have been incorporated here. In all nine shore sites and 19 sublittoral sites have been surveyed, From the data four littoral and 12 sublittoral habitat/community types are described, and 516 taxa listed. The shores were predominantly rocky and covered by fucoid algae. Within the two narrows enhanced currents yielded rich lower shore communities of sponges, hydroids and ascidians. The more exposed shores in the outer basin were not examined. The sediment shores supported populations of the lugworm Arenicola marina and the cockle Cerastoderma edule. Some extremely sheltered embayments had patches of the eelgrass Zostera nana and the unattached knotted wrack Ascophyllum nodosum ecad mackaii. In the sublittoral the sheltered middle basin had a narrow band of the kelp Laminaria saccharina which gave way to muddy plains at 2-3 m with populations of the seapen Virgularia mirabilis and the opisthobranch mollusc Philine aperta. In the outer loch rocky habitats extended to 5-12 m depth and supported a Laminaria hyperborea kelp forest, with L. saccharina on more unstable rocks. Below this sediment plains again supported populations of Virgularia mirabilis, but were accompanied here by the bivalve Arctica islandica, the heart urchin Echinocardium cordatum and the brittlestar Amphiura brachiata. Shallow sediments were covered by mats of filamentous algae. In the narrows there was a mixture of tideswept rock with L. hyperborea and Halidrys siliquosa and coarse gravels with sparse maerl, burrowing anemones and the sea cucumber Neopentadactyla mixta. For its size Loch Tarbert is subject to a particularly wide range of wave exposures, tidal currents and salinity. Habitat diversity within the loch is moderately high and species richness is high. Five of the habitat/community types and two species have been rated of conservation importance. The Cumhann Beag narrows is considered the most biologically interesting part of the loch and its shores are recommended for designation as a Site of Special Scientific Interest. Records currently considered sensitive have been removed from this dataset.
