Effect of harvesting, vegetation structure and composition on the abundance and demography of the land crab Cardisoma guanhumi in Puerto Rico

Abstract

Cardisoma guanhumi is the focus of an important artisanal fishery in Puerto Rico. Data on land crab landings point towards a dramatic decline in their abundance. This is cause for concern given the intrinsic value of the fishery and the important role these crabs play in coastal ecosystems. In this paper we examine the effect of harvesting and habitat quality on the abundance, survival, and size structure of C. guanhumi. To accomplish this we conducted a capture-mark-recapture study for a period of 18 months at three localities with minimal or no harvesting, and three with intense harvesting. Habitat quality at the six study sites was assessed by measuring vegetation composition-structure and litter biomass. We also conducted a leaf consumption experiment to evaluate leaf litter selectivity and limitation. Mean crab abundance differed significantly among sites, and this variation was significantly explained by differences in crab survival among sites. Sites with less harvesting tended to have higher survivorship and more crabs than sites where harvesting took place. Crabs mean size differed among study sites and was inversely related to abundance. Vegetation composition and structure, leaf litter standing stock and leaf-litter consumption differed among study sites. However, only some of these habitat characteristics are directly related to crab abundance or demography. There is a tendency for sites with a high leaf litter standing stock and large basal area to have the largest crabs but in low abundance. This study shows that C. guanhumi populations are very sensitive to increases in mortality that result from harvesting, and that the historical decline in abundance of this crab in Puerto Rico can be explained, in part, by an increase in trapping effort. Our results also indicate that C. guanhumi is a very plastic species, capable of occupying diverse types of coastal forests and a leaf litter generalist.

Site fidelity and population structure of blue land crabs (Cardisoma guanhumi Latreille, 1825) in a restricted-access mangrove area, analyzed using PIT tags

“…The level of low activity of C. guanhumi during the night period observed in this study agrees well with previous studies [18,53,54] that indicated the prevalence of C. guanhumi individuals collected mainly during the hours of the early morning and near sunset. The dietary habits of this species observed in this study agree with the observations made by Herreid [17].…”

“…However, some studies that used a marking method, such as Rodríguez-Fourquet and Sabat [53] that evaluated the impact of capture on the abundance, survival and demographics of C. guanhumi and Forsee and Albrecht [34], who estimated the population of C. guanhumi, both in Puerto Rico, reported at least one recapture event at the same first catch location. In this work, fidelity to the sites was evident, 77% of the individuals were recaptured in their area of origin, and the remaining individuals were recaptured in adjacent areas.…”

Long-term monitoring of crab Cardisoma guanhumi (Decapoda: Gecarcinidae) captures in Jobos Bay Estuary, Puerto Rico

“…The results of this long-term study indicate that the crab size did increase between 2004 and 2013 but since 2013 crab size has been steadily declining. This study and the results of Rodríguez-Fourquet and Sabat (2009) showed that crab hunting is taking place within NPA's in JBE and around Puerto Rico. While managers at the JBNERR have additional local management actions such as fencing, educational signs, removal of traps and increased patrolling to manage C. guanhumi within the NPA's, there has been little impact on the reduction of illegal activities such as overfishing (Jobos Bay Management Plan, 2017).…”

“…In Puerto Rico, land crab population numbers have been steadily declining since 1990's (Govender, 2007). Forsee & Albrecht (2012) and Rodríguez-Fourquet & Sabat (2009) found that the harvesting was high in Natural Protected Areas (NPA), possibly explaining declining crab size. Govender & Thomlinson (2010) found that drastic population declines were due to loss of habitat, which is principally mangroves.…”

Determination of the burrow shapes of Cardisoma guanhumi on Vieques, Puerto Rico

“…Crab size is an important variable in land management decisions because a given burrow area is usually occupied by similar-sized crabs [5]. As for burrow occupancy, it was once thought that there was only one crab occupying each burrow and only one opening for burrow access [12]. However, there is a conflicting report by Feliciano[4] that reported multiple occupants per burrow.…”