Comparative suitability of binocular observation, burrow counting and excavation for the quantification of the mangrove fiddler crab Uca annulipes (H. Milne Edwards)

Abstract

Quantification of mangrove crabs is notoriously difficult. Several techniques have been applied in the past, but have rarely been tested. This paper looks at the use of burrow counts (BUR), binocular counts (BIN) and excavation (EX) for quantifying the fiddler crab Uca annulipes (H. Milne Edwards). Fieldwork took place at Maruhubi mangrove (Zanzibar, Tanzania). Twelve 1 m² quadrats were assessed on one spring cycle and one neap cycle. The results showed a strong correlation between carapace width (CW) and burrow diameter (BD), namely CW (mm) = 1.37BD (mm) −0.64 (r ²=0.98; p<0.001; DF=73). The mean spring tide densities were 77.8 m⁻² (EX), 43.3 m⁻² (BIN) and 104.8 m⁻² (BUR). Of these, 37% (EX), 19% (BIN) and 42% (BUR) were juveniles (<7 mm CW). Census densities did not vary significantly between tides, although total BUR decreased from 1257 (spring) to 949 (neap). Whereas BIN significantly underestimated EX on both tides, BUR only significantly overestimated EX on the spring tide. Overall, burrow quantification matched EX better than binocular counts, suggesting the former is better for the quantification of U. annulipes. Whilst binocular census may be most appropriately carried out on spring tides, burrow quantification may be better on neap tides. A major reason for BIN underestimates and BUR overestimates were discrepancies in juvenile counts. Thus, 28% (spring) to 39% (neap) of adults, but 72% of excavated juveniles were unaccounted for by BIN. Spring tide BUR overestimated excavated adults by 25% but juveniles by 50%. Female BIN matched EX on the spring, but not the neap tide. Male binocular counts were significantly lower than EX on both tides. The potential influences on the results of crab size, sex, emigration and the method of sorting sediment during excavation are discussed.