Effective monitoring of populations and communities is a prerequisite for ecosystembased management of marine areas. However, monitoring programs often neglect important sources of error and thus can lead to biased estimates, spurious conclusions and false management actions. One such source of error is 'imperfect detectability', i.e. the inability of investigators to detect all individuals or all species in a surveyed area. Although there has been great effort to develop monitoring methods that account for imperfect detectability, the application of such methods in the marine environment is not as apparent as in other systems. Plot sampling is by far the most commonly applied method for biological monitoring in the marine environment, yet it largely ignores detectability issues. However, distance sampling, mark-recapture methods, repeated presence-absence surveys for occupancy estimation, and removal methods do estimate detection probabilities and provide unbiased estimates of state variables. We review these methods and the relevant tools for their application in studies on marine populations and communities, with the aim of assisting marine biologists and managers to understand the limitations and pitfalls associated with some approaches and to select the best available methods for their monitoring needs.

The relative vulnerability of various habitat types to Caulerpa racemosa var. cylindracea invasion was investigated in the National Marine Park of Zakynthos (Ionian Sea, Greece). The density of C. racemosa fronds was modelled with generalized additive models for location, scale and shape (GAMLSS), based on an information theory approach. The species was present in as much as 33% of 748 randomly placed quadrats, which documents its aggressive establishment in the area. The probability of presence of the alga within randomly placed 20 x 20 cm quadrats was 83% on 'matte morte' (zones of fibrous remnants of a former Posidonia oceanica bed), 69% on rocky bottoms, 86% along the margins of P. oceanica meadows, 10% on sandy/muddy substrates, and 6% within P. oceanica meadows. The high frond density on 'matte morte' and rocky bottoms indicates their high vulnerability. The lowest frond density was observed within P. oceanica meadows. However, on the margins of P. oceanica meadows and within gaps in fragmented meadows relative high C. racemosa densities were observed. Such gaps within meadows represent spots of high vulnerability to C. racemosa invasion.

The number of alien species in the Mediterranean Sea is increasing rapidly, but few attempts have been made to evaluate impacts on specific habitat types. The present study investigated alien biodiversity in Mediterranean marine caves, both by contributing new records of Eastern Mediterranean cave aliens, and by reviewing the scattered existing literature; the main goals were to highlight potential impacts and investigate the importance of cave environments for the expansion of alien species. Seven new alien species were found in marine caves of the Aegean and Ionian seas, raising the total number of aliens reported from Mediterranean marine caves to 56 species, classified as

The Natura 2000 network of protected areas aims to assure the long‐term survival of Europe's most valuable and threatened species and habitats. Yet, evidence shows that the present network fails to represent effectively the biodiversity of the region. Priority areas for conservation of coastal and offshore biodiversity features in the Greek Ionian Sea were identified, based on the principles of systematic conservation planning (SCP). SCP is a transparent method for the design of MPA networks and is considered more efficient and successful in representing the biodiversity of a region. The prioritization software Marxan was used and three scenarios with different sets of targets for 17 (high and low priority) conservation features were produced. These scenarios explicitly took into account socio‐economic factors expressed as a single cost metric, weighting different economic sectors in proportion to their contribution to the GDP of the region. Then results were compared with the existing Natura 2000 sites in terms of goal achievement, area requirements, and cost. The solutions produced by the systematic approach demanded less area and lower cost to achieve the goals set, when the selection of all Natura 2000 sites was not forced. Existing Natura 2000 sites alone failed to achieve conservation goals for some EU priority and other important coastal and offshore habitats and species of the Mediterranean Sea. It is suggested that the use of systematic conservation planning and related computational tools could benefit the selection of European marine priority areas, especially in the context of ecosystem‐based marine spatial management. Copyright © 2012 John Wiley & Sons, Ltd.