Larval connectivity in an effective network of marine protected areas

Abstract

Acceptance of marine protected areas (MPAs) as fishery and conservation tools has been hampered by lack of direct evidence that MPAs successfully seed unprotected areas with larvae of targeted species. For the first time, we present direct evidence of large-scale population connectivity within an existing and effective network of MPAs. A new parentage analysis identified four parent-offspring pairs from a large, exploited population of the coral-reef fish Zebrasoma flavescens in Hawai'i, revealing larval dispersal distances ranging from 15 to 184 km. In two cases, successful dispersal was from an MPA to unprotected sites. Given high adult abundances, the documentation of any parent-offspring pairs demonstrates that ecologically-relevant larval connectivity between reefs is substantial. All offspring settled at sites to the north of where they were spawned. Satellite altimetry and oceanographic models from relevant time periods indicated a cyclonic eddy that created prevailing northward currents between sites where parents and offspring were found. These findings empirically demonstrate the effectiveness of MPAs as useful conservation and management tools and further highlight the importance of coupling oceanographic, genetic, and ecological data to predict, validate and quantify larval connectivity among marine populations.

Figure 1. Adult yellow tang (Zebrasoma flavescens) photographed off the western (Kohala-Kona) coast of the Island of Hawai'i, where they occur at high densities.

Approximately half a million juvenile yellow tang (representing more than 1 million U.S. dollars) are collected from the island by the aquarium industry each year. Photo: W.J. Walsh.

Figure 2. Patterns of larval connectivity in yellow tang off the Island of Hawai'i as determined by direct detection of four different parent-offspring pairs.

Sample reefs are indicated by triangles and circles, where triangles represent marine protected areas (MPAs) and circles represent unprotected areas. The identified parents were sampled at Miloli'i and Punalu'u. Arrows point to the settlement site of the offspring. Solid lines indicate the first unequivocal evidence of an MPA seeding unprotected sites.

Figure 3. Sea surface height (cm, colors) as observed by satellites and the geostrophic velocity (cm/s, vectors) derived from satellite altimetry and averaged over the larval dispersal period of the four documented offspring shown in Figure 2 .

The cyclonic eddy is indicated by low sea surface heights and anti-clockwise rotation west of the Island of Hawai'i.

Figure 4. Dispersal of 1000 passive virtual drifters for 54 days – the pelagic larval duration of yellow tang – released from the two reefs where parents were identified.

Shown are passive dispersal of drifters released from Miloli'i on the date of spawning of a documented offspring (26 April 2006) at (A) sea surface level and (B) 30 meters below the sea surface, as well as passive dispersal of drifters released from Punalu'u at the date of spawning of another documented offspring (24 April 2006) at (C) sea surface level and (D) 30 meters below the sea surface. For drifters released at sea level, initial post-spawning dispersal was northward, followed by subsequent dispersal to the northwest. Drifters released at 30 meters below sea surface remained closer to the Island of Hawai'i and clustered near sites where offspring were identified. Note that yellow tang release floating gametes near the ocean surface and that the behavior of older larvae is presently unknown.