Biology of the subtropical sac-spawning euphausiid Nyctiphanes simplex in the northwestern seas of Mexico: Vertical and horizontal distribution patterns and seasonal variability of brood size

Vertical and horizontal distributions of the subtropical euphausiid juvenile and adult Nyctiphanes simplex were mapped from samples collected during winter and summer 2007 in the Gulf of California, Mexico. During winter, wide-ranging high densities occurred in most of the Gulf of California. Densities decreased considerably during summer, with only at few locations having high densities. N. simplex made short daily vertical migrations of <150 m with pronounced vertical seasonal ontogenetic segregation. In winter, most life stages, particularly gravid and ovigerous females, were found in the top 100 m at night, with temperatures <17 °C throughout the normal, cold, well-mixed water column. During summer, a seasonal thermocline at ∼50 m formed with temperatures between 22–29 °C above the thermocline. Reproductive females and males were found below the thermocline at >50 m, clearly avoiding layers with temperatures >20 °C. In both seasons, N. simplex occurred above the low-oxygen layer (<1.5 ml l−1), which occurred at 150 m during winter and 90 m during summer. In summer, this layer extended farther north and into shallower water columns than during winter. The low-oxygen layer acts as the bottom limit of vertical distribution and horizontal distribution is limited at the southern part of the gulf to temperatures >23 °C. Seasonal brood size and reproductive effort were estimated for both sides of the Baja California Peninsula under ship board experiments as a proxy of the relative effect of seasonal environmental conditions for euphausiid reproduction. Experiments were done during March, July, and December 2004 at the entrance to Bahía Magdalena and its westward continental shelf and in November 2005 and January and July 2007 in the Gulf of California. Contrary to broadcast-spawning euphausiids, N. simplex, a sac-spawning euphausiid, has a significant association of the brood size as a function of the total length of females. N. simplex produces an average brood of 52 eggs female–1 (range 5–116 eggs female–1) with a estimated total fecundity of 936 eggs female–1 in a life span (360–1337 eggs female–1), of which about 8% of its carbon weight is released per spawn, significantly higher than estimates of previous studies. In Bahía Magdalena, broods contained more embryos in March and July 2004 than in December 2004 when temperatures increased to >23 °C. In the Gulf of California, broods had higher numbers of embryos in November and July than in January 2007, suggesting that N. simplex has an out-of-phase reproductive season on both coasts of the peninsula. Reproductive investment effort was larger in the Gulf of California than in Bahía Magdalena, where females generated up to 18.3% of their weight in their broods, primarily by brood sizes produced from females between 10–12 mm total length that were particularly fecund during November and July.