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.

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This paper reports the weight-length relationship (WLR) for thirty-eight fish species from the Gulf of Castellammare (NW Sicily, Mediterranean Sea). A total of 59 870 fish were collected and measured during four seasonal trawl surveys carried out in 2005 at 10 to 200 m depth. The b parameter of the WLRs ranged from 1.79 (Cepola macrophthalma) to 3.56 (Conger conger). Significant differences in both a and b values among seasons were detected for eight species. Moreover, the condition factors (as weight-at-length) of Merluccius merluccius and Mullus barbatus were compared in different area ⁄ -year combinations characterized by the presence or absence of trawling impact. While M. barbatus showed a higher condition factor in trawled areas ⁄ years, possibly due to density-dependent factors linked to the increased biomass in the no-trawl area, no such link was detected in M. merluccius.

This paper presents the results of an application of ultrasonic telemetry on white seabream, Diplodus sargus inhabiting an artificial reef (AR) in NW Sicily (western Mediterranean Sea). The objective of the study was to investigate the movement pattern of seabreams, verify their homing behaviour and site fidelity, determine their home range and describe their use of the habitat. Four seabreams were tagged and released, and their movements were recorded with automated and manual acoustic receivers. The spatial and temporal distribution of positional data suggest that the tagged seabreams hide inside the AR during the day, staying out of their shelter at night. The nocturnal movements of the tagged fishes are suggested to be a search for food in the seagrass patches surrounding the ARs. The monitored seabreams showed clear homing behaviour and strong site fidelity. Their home range extended from 0.01 to 0.17 km and included the AR and the surrounding sandy area with seagrass patches. Home range areas increased proportionally to the distance between the refuge on AR and the foraging areas on seagrass patches. The higher activity of seabreams during the night was interpreted as a result of a trade-off between predation risk and foraging needs.

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No-take zones (NTZs) have been shown to be useful tools for marine conservation and fishery management, although the lack of information on species' movements often makes it difficult to properly establish NTZ size. Using acoustic telemetry techniques, we monitored the movements, home range (HR) and homing ability (to capture sites) of 22 adult white seabream Diplodus sargus sargus in a fully protected portion (138.60 ha) of the Torre Guaceto Marine Protected Area (SE Italy). After release at a different location than the site of capture, 85% of the tagged fish returned to the capture site within 3 d. Fish were monitored for 161 d. All tagged fish spent most of the time within the monitoring area (fish presence index = 92.8%) and showed a mean HR of 20.6 ha. These results indicate that the studied NTZ effectively protects seabream, as it entirely encompasses their HRs, which are on average far smaller than the reserve. Twelve individuals left the monitoring area during the period of the year that corresponds to their known time of spawning. This potential emigration during the spawning period indicates that the reserve alone does not fully protect white seabream and that other management options, such as a seasonal fishing closure during the reproductive period, may be needed. Estimates of movement patterns and HRs of fishes, therefore, represent useful information to better understand, refine and enhance the value of NTZs for protecting ecologically valuable species.

Marine reserves and restocking initiatives are sometimes used as a tool to enhance spiny lobster stocks. In such initiatives it is crucial to follow the movement of lobsters once they are released at sea in restocking experiments.This paper presents the results of the application of an ultrasonic telemetry system to the monitoring of 11 lobsters (90 AE14 mm mean carapace length) released in the Capo Gallo-Isola delle Femmine Marine Reserve in north-western Sicily, central Mediterranean. The system comprised transmitters glued onto the lobster carapace, and manual as well as automated receivers to locate tagged animals. The data (i.e. number of detected signals) were tested to assess any di¡erence in the diel activity of lobsters. The ¢eld study lasted 79 days in total. Lobsters remained in the study area for periods ranging from a few hours to the entire duration of the study. The longest distance travelled by a tagged lobster was 2.2 km. The number of signals varied signi¢cantly across the day, with the highest value recorded in the full-light hours (1000^1500), but they were not su⁄cient to assess clearly the diel activity of the released lobsters.The interpretation of data suggests that: (1) released lobsters preferred a deeper habitat than that of the release site; and (2) the lobsters that reached a rough rocky area with available shelters settled there, while those that did not meet such a habitat soon moved out of hydrophone detecting range.