Plucking or cutting Gelidium sesquipedale? A demographic simulation of harvest impact using a population projection matrix model

Abstract

A matrix model describes the annual dynamics of a commercial (harvested by plucking) Gelidium sesquipedale population off Cape Espichel, Portugal. Vital rates were measured from a frond population divided into size classes; annual transition probabilities among them were calculated. Transition probabilities under harvest by cutting are derived by assuming that all harvested fronds are cut to the first size class, and none are plucked. Simulations of the annual population dynamics for harvest by both plucking and cutting are used to assess which harvest strategy will optimize yields. Assuming the same efficiency for both strategies, cutting fronds to 7 cm (as mechanical harvesters do) results in a higher population growth rate (λ = 1.08 to 1.35) than occurs with the plucking technique (λ = 0.85). Simulations of population recovery show the number of fronds in each size class available the next harvest season will be higher when cut than plucked. This model can also optimize yields by predicting the more efficient season opening, and harvesting cutting height.