Description of first nursery area for a pygmy devil ray species (Mobula munkiana) in the Gulf of California, Mexico

Abstract

Munk's pygmy devil rays (Mobula munkiana) are medium-size, zooplanktivorous filter feeding, elasmobranchs characterized by aggregative behavior, low fecundity and delayed reproduction. These traits make them susceptible to targeted and by-catch fisheries and are listed as Vulnerable on the IUCN Red List. Multiple studies have examined fisheries impacts, but nursery areas or foraging neonate and juvenile concentrations have not been examined. This study describes the first nursery area for M. munkiana at Espiritu Santo Archipelago, Mexico. We examined spatial use of a shallow bay during 22 consecutive months in relation to environmental patterns using traditional tagging (n = 95) and acoustic telemetry (n = 7). Neonates and juveniles comprised 84% of tagged individuals and their residency index was significantly greater inside than outside the bay; spending a maximum of 145 consecutive days within the bay. Observations of near-term pregnant females, mating behavior, and neonates indicate an April to June pupping period. Anecdotal photograph review indicated that the nursery area is used by neonates and juveniles across years. These findings confirm, for the first time, the existence of nursery areas for Munk's pygmy devil rays and the potential importance of shallow bays during early life stages for the conservation of this species.

Figure 1

(a) Mexican Pacific and Baja California Peninsula. The yellow square located on the southwestern portion of Gulf of California outlines the location of (b) La Paz Bay and the surrounding islands of San Jose (SJI), Cerralvo (CI), and the Espiritu Santo Archipelago (ESA) outlined in yellow dashed square. The receiver locations (n = 21) are indicated with black–white dots. (c) Mobula munkiana early life stage individuals aggregate in the shallow bay of Ensenada Grande outlined with a yellow square. The map was created using Surface Mapping System (Golden Software, Inc., 1993–2012, https://www.goldensoftware.com/products/surfer) and the coastline data was extracted from GEODAS-NG (National Geophysical Data Center, 2000).

Figure 2

(a) Capture locations of M. munkiana between August 2017 and June 2018 at Ensenada Grande. Circle size indicates the number of individuals captured at each location by life stage. Numbers indicates the bathymetric lines. (b) Number of M. munkiana captured at Ensenada Grande per month and life stage following the same color code.

Figure 3

(a) Mobula munkiana detection map between August 2017 and May 2019 at La Paz Bay and the surrounding islands of San Jose (SJI), Cerralvo (CI), and (b) the Espiritu Santo Archipelago (ESA). The proportion (%) is indicated by circle size and color for each receiver described in the legend.

Figure 4

Mobula munkiana (n = 7) detections recorded at Espiritu Santo Archipelago between August 2017 and May 2019. Left axis specified code of the animal tag–disc width in centimeters–Sex (F: female; M: male). Ensenada Grande receivers are indicated in red (Inshore) and the rest of the Espiritu Santo Archipelago array is in orange (Offshore). Black line represents the temperature at Ensenada Grande from August 2017 to April 2018. Months of mating and pupping season are indicated in yellow.

Figure 5

(a) Receiver locations (black–white dots: RS1 at 5 m depth and RS2 at 26 m depth) and zooplankton sampling stations (orange lines) locations at Ensenada Grande. Circular plots of detections per hour of acoustic tagged M. munkiana (n = 7) at each receiver. (b) Zooplankton mean biovolume (mL 100 m⁻³) and standard error at the three sampling stations collected at day and night during the study period.

Figure 6

Juvenile males M. munkiana with undeveloped claspers (indicated by arrows) at Ensenada Grande during recreational dives in (a) November 2013, (b) October 2014 and (c) October 2016. Images copyright: (a), (b) Erick Higuera and (c) Luke Inman.