Survey results of two populations of Carcinoscorpius rotundicauda at a fishing ground and within a protected area were compared so as to establish effects of human activities on the species. Tachypleus tridentatus and Tachypleus gigas were also found in both sites but in substantially less abundance. The operational sex ratios were male-biased at both sites; 5.50 (n = 52) at the fishing ground (Site 1) and 2.58 (n = 68) at the protected area (Site 2). Size distributions at both sites were similar of which females were approximately 16% larger than males. A captive experiment was conducted where pair-forming behaviour of C. rotundicauda was observed for 30 days. Amplexus were most frequently formed when the sex ratio was balanced, lasting for 2.44 ± 2.03 days, and eight days maximum. Female body size and amplex-forming frequency were positively correlated ( r = 0.678, n = 7, p = 0.10), attributable to fitness projected by the phenotype. Solitary males did not exhibit aggression towards paired males, allowing the latter to retain mate exclusivity for considerable periods. Anthropogenic pressures caused an unnatural shift in the population structure of local horseshoe crabs that could lower their reproductive success, making eventual extirpation possible.