Predação humana no litoral rochoso alentejano - Universidade de ...

not been observed in the yield by shellfish collection and angling obtained during low tide.
Mean weight of catches per fisher of wrasses (Symphodus spp.), bogue (Boops boops)
and white seabream (Diplodus sargus) were responsible for most variation between
shores exposed to different intensity of human exploitation, and catches of wrasses and
bogue were bigger on shores exposed to lower exploitation intensity. Mean value of fresh
weight of catches obtained per fisher during low tide, and per angler during thirty minutes
of flood tide, were 2.2 and 0.2kg, respectively. Extrapolating yield values obtained in this
analysis and using the data from the study of predation intensity, the annual global yield
was estimated as 4.3 tonnes of fresh weight per kilometre of coastline. The annual yield of
shellfish collection and angling was estimated as 2.4 and 1.4 tonnes of fresh weight per
kilometre of coastline, respectively. Using these estimates and results from other studies,
the sustainability of the exploitation of the purple sea urchin Paracentrotus lividus, the
stalked barnacle Pollicipes pollicipes and the limpet Patella ulyssiponensis was analysed.
The present exploitation rate of P. lividus is not a risk for its conservation. The stock of P.
pollicipes was considered to be highly to fully fished. The stock of P. ulyssiponensis was
considered to be moderately fished.
Limpet exploitation may have negative effects on populations with consequences
for communities structure and functioning. Thus, the analysis of the ecological impact of
human predation activities on rocky shores of Alentejo was made through the description
of patterns of variation of limpets abundance, size and vital area (feeding territory) on
shores exposed to different exploitation intensity. In parallel, a manipulative experiment
tested the effects of limpets removal. The survey analysed spatial and temporal variation
at different scales (shore and site, period and date), using two tide levels, shores exposed
to higher and lower exploitation intensity and two periods – before and after summer. The
spatial impact of the human predation was detected at the variance per site of the size of
exploited limpets and the vital area of limpets on the low shore. This small scale variability
was significantly higher on shores exposed to higher exploitation intensity; it was only
observed after summer. The temporal impact of the human predation was detected at the
variance per site of the abundance and size of exploited limpets in low shore; this small
scale variability was significantly higher after summer.
The manipulative experiment on the effects of limpets removal was made on the
low shore, using different spatial scales and three treatments: no limpets removal (C),
large limpets removal (G), and all limpets removal (T). Establishment of the experiment
and first sampling was made in summer/autumn (t0) and ca. seven months later second
sampling (t1) was made. Smallest spatial scales used were territory (mean initial total
area of 42,1cm 2 ) and area (1x2m). When area was the maximum spatial resolution,
significant effects of limpet removal were detected in the abundance of large limpets
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