Eastern Tropical Pacific Off Research Articles

The Panama Low Level Jet: extension, annual cycle and modes of variation

The Panama Low Level Jet (PLLJ) is the inter-isthmic jet closest to the equator, whose activation modifies circulation, sea surface temperature and climate in most of the Eastern Tropical Pacific off Colombia. Through the analysis of 26 years (1992-2018) of sea wind data, the extent of the jet and its main frequencies of variability were determined, in addition to its relationship with phenomena of climatic variability. The area of direct influence of the jet was estimated to be 59295.34 km², being located between 4.63° and 9oN. The PLLJ is activated annually during the boreal summer (December-April) with intense intra-seasonal variability (with frequencies between 1 to 5 months) and inter annual perturbations associated with El Nino Southern Oscillation events. During warm ENSO events, the PLLJ activity last ~ 2 months more (from mid-November to the end of May) and its spatial extent increases southerly reaching ~ 2 ° N. Additionally, during warm ENSO years, there is an extemporaneous activation of PLLJ in the boreal summer, possibly linked to the intensification of the North Trade Winds over the Caribbean basin in the middle of the year and the increase in the north-south pressure gradient through the Panama isthmus during intense El Nino events. The entry of Caribbean winds in unusual seasons changes the climatology of the PLLJ and increasing its influence in the Eastern Tropical Pacific during the ENSO warm phases.

Seasonal variability of stable isotope values and niche size in tropical calanoid copepods and zooplankton size fractions

The seasonal variability of stable isotope values (δ15N and δ13C), ∆15N values, and niche width of calanoid copepod species, three zooplankton size fractions and seston were assessed from samples obtained monthly between January and December 2012 at a coastal sampling site in the Eastern Tropical Pacific off Mexico (19°09′03″ N, 104°44′50″ W). Twenty calanoid copepod species were analyzed, 15 of which were sampled in both the cool, productive ‘mixed’ period and the warm, oligotrophic ‘stratified’ period. While copepod δ15N and ∆15N values were higher during the mixed period, seston δ15N values did not differ between periods. The δ13C values indicated a seasonal difference between a priori-defined groups of oceanic and coastal copepod species. The seasonal variations of both the isotopic niche (Bayesian ellipses) values and Layman metrics in tropical copepods and zooplankton fractions suggested an opportunistic feeding strategy responding to the local coupled physical–biological seasonal processes. During the productive mixed period, the trophic web was more robust and stable, and a wider niche was observed. Contrary to commonly reported responses to environmental conditions, we found a more complex mesozooplankton food web during the productive period and a shorter web during the oligotrophic period, highlighting the importance of researching trophic dynamics in coastal tropical ecosystems.

Structure of euphausiid assemblages in the Eastern Tropical Pacific off Colombia during El Niño, La Niña and Neutral conditions

In order to understand how the structure and composition of euphausiid assemblages are affected by changes in environmental conditions caused by El Niño and La Niña events in the Colombian Pacific, euphausiids present in zooplankton samples obtained during Estudio Regional del Fenómeno El Niño – Cuenca Pacífica Colombiana (ERFEN-CPC) cruises, in September 2009 (El Niño), October 2011 (La Niña) and September 2012 (Neutral) were analyzed. Fifteen species (5 genera) were identified, of which Euphausia diomedeae was the most abundant during El Niño and Euphausia distinguenda was the most abundant during La Niña and the Neutral condition. In addition, considering all euphausiid species found in this study; under La Niña and Neutral conditions, juveniles were the most abundant phase; while during El Niño, furcilia larvae were the most abundant. During the Neutral condition, the total abundance of euphausiids was significantly higher than during El Niño and La Niña, suggesting that the environmental changes caused by these anomalous conditions would affect abundance due to possible movements of the species towards more favourable conditions. Finally, during El Niño, the euphausiid assemblage was significantly different from the other two conditions, suggesting that the increase in temperature produced by this condition affect the structure and composition of euphausiids assemblage in the Colombian Pacific.

Vertical variability ofEuphausia distinguendametabolic rates during diel migration into the oxygen minimum zone of the Eastern Tropical Pacific off Mexico

Vertical variability of<i>Euphausia distinguenda</i>metabolic rates during diel migration into the oxygen minimum zone of the Eastern Tropical Pacific off Mexico

Seasonal variability of copepod community structure and abundance modified by the El Niño-La Niña transition (2010) in the tropical Pacific off central Mexico

Seasonal variability of the community structure of copepods in the Eastern tropical Pacific off central Mexico was studied during three distinct hydrodynamic periods in 2010 using statistical and multivariate analyses. The survey period included the second half of the 2009-2010 El Niño (January), the neutral transition period (May-June), and the first half of the 2010-2011 La Niña (October). Seventy-eight copepod species were identified; richness ranged from 11 to 47 species per station, with seasonal averages from 25 species in May to 35 species in January. Cluster analysis indicated that there were four principal groups present across the surveyed periods, defined by January (El Niño), October (La Niña), May offshore stations, and May upwelling stations (cyclonic eddy and coastal stations). There were no significant differences in abundance, but the January (El Niño) cluster was most diverse with 32 species, May offshore and October (La Niña) clusters each had 25 species, and the May upwelling was the least diverse cluster with 18 species. Mesoscale processes were strongest during May, which was the only period with a significant inshore-offshore gradient of species richness and diversity. Canonical correspondence analysis (CCA) revealed that variability was primarily driven by subsurface (75-200 m) ammonium, and surface (0-50 m) temperature, nitrates+nitrites, salinity and phosphorus. Copepodites and adults of the primarily herbivorous Eucalanidae dominated the stations of the upwelling cluster, while copepodites and adults of the carnivorous Euchaetidae dominated the January (El Niño) station cluster. The higher Chl a levels during the less productive (reduced upwelling) El Niño period were probably due to reduced grazing activities and increased ammonium availability through increased zooplankton metabolism. The horizontal distribution of copepods in the Eastern Tropical Pacific off Mexico appears to be principally defined by mesoscale eddy processes (offshore) and upwelling (coastal). These mesoscale processes were affected by El Niño - La Niña transitions, which subsequently disrupted the inshore-offshore gradient and in the case of El Niño likely caused reductions in copepod abundance across the entire region which persisted for the entire study period, and possibly longer.

Annual egg production rates of calanoid copepod species on the continental shelf of the Eastern Tropical Pacific off Mexico

We provide the first estimations of calanoid copepod egg production rates (EPR) in the Eastern Tropical Pacific over an annual cycle (January–December 2011). Gravid females were collected twice monthly and incubated for 12 h without food to estimate EPR, weight-specific fecundity (Gf), spawning success (SS, percentage of females to spawn out of the total species incubated per month and season) and egg hatching success (EHS). This study reports the average EPR of 10 species and the monthly EPR and Gf of four planktonic calanoid copepods (Centropages furcatus, Temora discaudata, Pontellina sobrina, and Nannocalanus minor) that spawned with enough frequency to infer their seasonal reproductive patterns. These species showed distinct seasonal reproductive strategies. Most copepod species spawned sporadically with large EPR variability, while three copepod species reproduced throughout the year (C. furcatus, T. discaudata and P. sobrina) and N. minor spawned only during the mixed period (Feb–May). The four species had relatively similar average EPR (C. furcatus 16, T. discaudata 18, P. sobrina 13, and N. minor 12 eggs fem−1 day−1). These are the first EPR estimations of P. sobrina and its previously known reproductive period is expanded. A Canonical Correspondence Analysis (CCA) was used to analyze EPR and species abundance of all calanoid copepods (40 spp.) collected throughout the time series in relation to temperature, salinity, mixed layer depth (MLD), dissolved oxygen, and chlorophyll a (Chl-a) concentrations to identify the variables that best explained the copepod abundance variability. Temperature, Chl-a, and salinity had the strongest effect on the biological variables, linked to seasonal and episodic upwelling-downwelling processes in the surveyed area. As a result of moderate upwelling events and seasonal variation of environmental conditions, it appears relatively few species are capable of maintaining continuous reproduction under the relatively higher temperatures and strong fluctuations of food availability that exist in this coastal habitat of the Eastern Tropical Pacific.

Role of circulation scales and water mass distributions on larval fish habitats in the Eastern Tropical Pacific off Mexico

AbstractOn the basis of five oceanographic cruises carried out in the Eastern Tropical Pacific off Mexico, relationships between the larval fish habitats (areas inhabited by larval fish assemblages) and the environmental circulation scales (mesoscale, seasonal, and interannual) were examined. Analysis of in situ data over a grid of hydrographic stations and oblique zooplankton hauls with bongo net (505 µm) was combined with orthogonal robust functions decomposition applied to altimetry anomalies obtained from satellite. During both cool (March and June) and warm (August and November) periods, Bray‐Curtis dissimilarity Index defined three recurrent larval fish habitats which varied in species composition and extent as a function of the environmental scales. The variability of the Tropical larval fish habitat (characterized by high species richness, and dominated by Vinciguerria lucetia, Diogenichthys laternatus, and Diaphus pacificus) was associated with the seasonal changes. The Transitional‐California Current larval fish habitat (dominated by V. lucetia and D. laternatus, with lower mean abundance and lower species richness than in the Tropical habitat) and Coastal‐and‐Upwelling larval fish habitat (dominated by Bregmaceros bathymaster) was associated mainly with mesoscale activity induced by eddies and with coastal upwelling. During February 2010, the Tropical larval fish habitat predominated offshore and the Transitional‐California Current larval fish habitat was not present, which we attribute to the effect of El Niño conditions. Thus, the mesoscale, seasonal, and interannual environmental scales affect the composition and extension of larval fish habitats.

Three-dimensional distribution of larval fish habitats in the shallow oxygen minimum zone in the eastern tropical Pacific Ocean off Mexico

Three-dimensional distribution of larval fish habitats was analyzed, from the upper limit of the shallow oxygen minimum zone (~0.2mL/L) to the sea surface, in the eastern tropical Pacific Ocean off Mexico in February 2010. The upper limit rises from ~250m depth in the entrance of the Gulf of California to ~80m depth off Cabo Corrientes. Three larval fish habitats were defined statistically: (i) a Gulf of California habitat dominated by Anchoa spp. larvae (epipelagic species), constrained to the oxygenated surface layer (>3.5mL/L) in and above the thermocline (~60m depth), and separated by a salinity front from the Tropical Pacific habitat; (ii) a Tropical Pacific habitat, dominated by Vinciguerria lucetia larvae (mesopelagic species), located throughout the sampled water column, but with the highest abundance in the oxygenated upper layer above the thermocline; (iii) an Oxygen Minimum habitat defined mostly below the thermocline in hypoxic (<1mL/L; ~70m depth) and anoxic (<0.2mL/L; ~80m depth) water off Cabo Corrientes. This subsurface hypoxic habitat had the highest species richness and larval abundance, with dominance of Bregmaceros bathymaster, an endemic neritic pelagic species; which was an unexpected result. This may be associated with the shoaling of the upper limit of the shallow oxygen minimum zone near the coast, a result of the strong costal upwelling detected by the Bakun Index. In this region of strong and semi-continuous coastal upwelling in the eastern tropical Pacific off Mexico, the shallow hypoxic water does not have dramatic effects on the total larval fish abundance but appears to affect species composition.

Air-sea CO2fluxes above the stratified oxygen minimum zone in the coastal region off Mexico

Oxygen minimum zones (OMZs) are important sources of CO2 to the atmosphere when physical forces bring subsurface water with high dissolved inorganic carbon (DIC) to the surface. This study examines, for the first time, the influence of the OMZ of the coastal North Eastern Tropical Pacific off Mexico on surface CO2 fluxes. We use variations in the oxycline depth and subsurface water masses to discern physical oceanographic influences. During two cruises, in November 2009 and August 2010, DIC and total alkalinity (TA) measurements were used to estimate pCO2 and air-sea CO2 fluxes. At the OMZ layer, Subtropical Subsurface Water (StSsW) was found to have high pCO2 values (1290 ± 70 μatm). Due to strong vertical stratification, however, the relationship between ΔpCO2 at the air-sea interface and the oxycline/StSsW upper limit depth was weak. During November, the region was a weak source of CO2 to the atmosphere (up to 2.5 mmol C m−2 d−1), while during August a range of values were observed between −4.4 and 3.3 mmol C m−2 d−1). Strong stratification (>1200 J m−3) prevented subsurface mixing of water from the OMZ to the upper layer; particularly in November 2009 which was during an El Niño event. Results suggest that advection of surface water masses, reinforced by strong vertical stratification, controlled surface pCO2, and air-sea CO2 fluxes.

The mechanisms involved in defining the northern boundary of the shallow oxygen minimum zone in the eastern tropical Pacific Ocean off Mexico

We identify the dynamic northern boundary of the shallow Oxygen Minimum Zone (OMZ) in the eastern tropical Pacific off Mexico (ETPM; 16–23°N). Spatial and temporal variability of the upper limit of the OMZ (DO=9μmolL−1) is studied analyzing the World Ocean Database 2009 (WOD09) and regional data collected during nine oceanographic surveys (2002–2010). The mean depth of the upper limit of the OMZ ranges from 300 to 400m from 20° to 23°N, where the California Current induces the advection of oxygen-rich water. Further south (from 16° to 20°N), the shallow upper limit of the OMZ (∼60m) results from the poleward transport of hypoxic Subtropical Subsurface Water (StSsW) near the surface layer (∼25.6kgm−3). A transitional zone is located in the north (20–23°N), where the influence of StSsW disappears, and the top of the OMZ is forced deeper by eddies generating southward intrusion of California Current Water (CCW). This oxygen-rich water (50–150μmolL−1), transported overlying the 18°C isotherm, defines the northern boundary of the shallow OMZ in the ETPM. The mechanisms involved in the definition of the northward distribution of the shallow OMZ are associated with seasonal advection of CCW and StSsW as well as the intensification of regional mesoscale circulation. Regional and temporal dynamics of these two subsurface water masses largely determine the position of the northern boundary of the shallow OMZ in the ETPM.

The effects of a lanthanide metal alloy on shark catch rates

Bycatch of sharks in longline fisheries has contributed to declines in shark populations and prompted the need for exploring novel technologies to reduce the incidental capture of sharks. One potential strategy is to exploit the unique electrosensory system of sharks, used to detect weak electric fields. Metals from the lanthanide series, made up of neodymium (Nd) and praseodymium (Pr), produce strong electric fields in water. In this study, we tested the effects of an Nd/Pr alloy on shark catch rates. Using longline fishing gear, we compared the catch rates of baited hooks affixed with either a block of the metal alloy (experimental) or a lead weight (control). Four experiments were conducted in different regions of the Pacific Ocean. Two bottom longline experiments were conducted inside and offshore of Kaneohe Bay, Hawaii. One of these experiments targeted young of the year scalloped hammerhead sharks (Sphyrna lewini), while the other targeted sandbar (Carcharhinus plumbeus) and tiger sharks (Galeocerdo cuvier). In the Southern California Bight (SCB), pelagic longlines were deployed to target mako (Isurus oxyrinchus) and blue sharks (Prionace glauca) and longlines targeting pelagic sharks were set in the Eastern Tropical Pacific (ETP) off Ecuador. There was a significant reduction in juvenile hammerhead sharks caught on hooks with the lanthanide metal compared to the controls. In contrast, there was no difference in the catch rates for experiments targeting sandbar sharks in Hawaii or those conducted in the SCB and Ecuador. These results suggest that there are inter-specific differences regarding the effects of lanthanide metals on catch rates. This may reflect the diverse feeding strategies and sensory modalities used by shark species for detecting and attacking prey.

Growth, maturity, and size-at-age variation of the bigheaded shrimp Heterocarpus vicarius (Decapoda, Pandalidae) in the Eastern Tropical Pacific off Colombia

The bigheaded shrimp, Heterocarpus vicarius, is a commercially important species fished by bottom-trawling fleets in the Colombian Pacific. However, the life-history parameters necessary for a proper analysis of the population dynamics of this species remain unknown. This paper studies the growth, maturity, and size-at-age variations, of Heterocarpus vicarious, as well as observations on recruitment patterns. The Von Bertalanffy growth parameters were estimated by sex through modal progression analysis, which consisted of grouping cohorts into age classes. Growth parameters were similar for males and females, without significant differences (F = 3.23, p > 0 . 05 ). The growth of both sexes is described by the following parameters: L ∞ = 15 . 26 ± 1 . 93 cm total length (TL), K = 0 . 594 ± 0 . 24 yr − 1 , and t 0 = − 0 . 66 ± 0 . 31 yr . Size-at-age variability was due mainly to processes occurring before larval settlement, since the mean length of same-age individuals born during different reproductive events (intercohort) varied more than that of individuals born during the same reproductive event (intracohort). Length at 50% maturity was estimated at 11.72 cm TL for females, with 95% confidence intervals between 10.57 and 13.71 cm TL. The growth curve and the relatively advanced age at maturity estimated for H. vicarius in the study area suggest the species is very vulnerable to fishing exploitation.

Structure of hyperiid amphipod assemblages on Isla Gorgona, eastern tropical Pacific off Colombia

Temporal variation of hyperiid amphipod structure assemblages was studied on Isla Gorgona, eastern tropical Pacific (ETP) off Colombia between September 2005 and August 2006. Forty-six species were found during the entire sampling period:Hyperioides sibaginis, Lestrigonus bengalensis, Phronimopsis spinifera, Tetrathyrus forcipatusandParalycaea gracilisdominated the assemblage, representing 92%. The dendogram based on the Bray–Curtis similarity index showed that the hyperiid assemblages were separated into two groups, which did not coincide with the seasonality described for the ETP (wet versus dry season). Rather, groups comprised wet (May to November) and dry season months (December to April). The first group included November, December, February and March. During these months, significantly higher hyperiid richness, diversity and abundance were found, although colder subsurface water temperatures in Gorgona were registered only during February and March. The other group included May to October and January, and was characterized by lower hyperiid diversity and abundance values. Significant correlations were found between hyperiid abundance and the possible gelatinous zooplankton hosts (medusae, siphonophores and salps). This study contributes to increasing knowledge of the zooplankton community in the Panama Bight, as well as of a poorly studied group in the ETP.

Ecological patterns, distribution and population structure of Prionace glauca (Chondrichthyes: Carcharhinidae) in the tropical-subtropical transition zone of the north-eastern Pacific

Regional ecological patterns, distribution and population structure of Prionace glauca were analyzed based on samples collected on-board two long-line fleets operating in oceanic waters (1994–96/2000–02) and in coastal oceanic waters (2003–2009) of the eastern tropical Pacific off México. Generalized additive models were applied to catch per unit of effort data to evaluate the effect of spatial, temporal and environmental factors on the horizontal distribution of the life stages (juvenile, adult) and the sexes at the estimated depth of catch. The presence of breeding areas was explored. The population structure was characterized by the presence of juveniles' aggregations and pregnant females towards coastal waters and the presence of adult males' aggregations towards oceanic waters. The species exhibited horizontal segregation by sex-size and vertical segregation by sex. Distribution of the sex-size groups at oceanic waters was seasonally affected by the latitude; however, at coastal oceanic waters mainly females were influenced by the longitude. Latitudinal changes on the horizontal distribution were coupled to the seasonal forward and backward of water masses through the study area. Adult males showed positive relationship with high temperatures and high-salinities waters (17.0°–20.0 °C; 34.2–34.4) although they were also detected in low-salinities waters. The distribution of juvenile males mainly occurred beyond low temperatures and low-salinities waters (14.0°–15.0 °C; 33.6–34.1), suggesting a wide tolerance of adult males to explore subartic and subtropical waters. At oceanic areas, adult females were aggregated towards latitudes <25.0°N, mainly associated to subtropical waters during summer. The distribution of juvenile females indicated its preference by lower temperatures and more saline waters. Presence of pregnant females suggests that the eastern tropical Pacific off México represents an ecological key region to the reproductive cycle of P. glauca.

Effect of the oxygen minimum zone on the second chlorophyll maximum

Field data collected during June 2005 were used to determine the relationship between the second fluorescence maximum (FMax), the top of the oxygen minimum zone (OMZ), and physical processes (coastal currents, eddies, and upwelling) in the northern region of the Eastern Tropical Pacific off Mexico (ETPM). A recurrent second FMax was observed in the ETPM, which was formed only when the upper limit of the OMZ (9.0 µmol L–1) overlapped with the 1% downwelling blue irradiance depth (Ed490). The presence of the second FMax increased the total integrated water column chlorophyll from 20% to 40%. The second FMax was absent from areas where oxygenated California Current Water (CCW) deepened the upper limit of the OMZ below 1% Ed490. The poleward Mexican Coastal Current carried less oxygenated Subtropical Subsurface Water into the area, and enabled the formation of the second FMax. The variability of the second FMax driven by mesoscale physical processes was related to coastal upwelling and cyclonic eddies only in areas not influenced by CCW.

Primary productivity observations in the eastern tropical Pacific off Cabo Corrientes, Mexico

The in situ primary productivity (PP) and phytoplankton biomass (chlorophyll a concentration, Chla) of the Cabo Corrientes region, in the eastern tropical Pacific off Mexico, were studied using data from three oceanographic cruises (May and November 2002, and June 2003). The coastal region (&lt;60 km) was found to be affected by coastal upwelling, causing phytoplankton biomass and production rates in this region to be up to two times higher than at the offshore stations. The highest surface Chla values were registered in May 2002 (&gt;2 mg m–3), when the mean PP was 361 mgC m–2 d–1. The highest PP values were recorded in June 2003, with a mean value of 447 mgC m–2 d–1, and the lowest in November 2002, with a mean value of 200 mgC m–2 d–1. Satellite ocean color images, supported by our field data, suggest that the area off Cabo Corrientes can be characterized by three periods: (1) a relatively intense upwelling period, with high Chla and PP (spring); (2) an upwelling relaxation period, when the highest PP values of the year were recorded (late spring-early summer); and (3) a summer-fall period, with strong stratification and lowest Chla and PP values. The high PP registered for the region off Cabo Corrientes during the spring season leads us to conclude that this is an area of high fertility, similar to other rich regions in the Pacific Ocean off Mexico.

Confirmation of the spawning area of the striped marlin, Tetrapturus audax, in the so‐called core area of the eastern tropical Pacific off Mexico

We report the presence of striped marlin larvae in coastal waters of the mouth of the Gulf of California, collected during seven oceanographic cruises during 1990 to 1993. From 169 sampled stations, 30 produced a total of 68 larvae of 2.4–30.5 mm total length. Most of the larvae were about 3 mm, indicating a recent spawn. The sea surface temperature at the sampled stations ranged from 19.4°C to 32.0°C, but larvae were caught at stations with temperatures between 27.8°C and 31.5°C. These results clarify the existence of a spawning area of striped marlin in the eastern Pacific, located closer to the coast than previously believed, during several months of the year.

Unusual Larval Growth Production of Nyctiphanes simplex in Bahía de La Paz, Baja California sur, México

ABSTRACT Nyctiphanes simplex is one of the few neritic euphausiid species in the world. This species frequently dominates the nearshore waters of the Gulf of California, the transition zone of the California Current, and the eastern tropical Pacific off Ecuador and Peru. It has a high production in coastal and upwelling waters and plays an important role in the trophodynamics of these areas. This work examines the larval growth production (Pg) of Nyctiphanes simplex in Bahfa de La Paz, Baja California Sur, Mexico. The data were collected during 7 oceanographic expeditions from February-August 1990. Except for August, larval growth production remained at high levels in the bay. Daily and annual growth production for both transects were estimated. The mean annual larval Pg value for the entire area was 195.7 mg·dw·m-3 yr-1, the highest value recorded for the genus Nyctiphanes. The annual mean P:B ratio (defined as the ratio of annual production to average annual biomass for a particular species) for the bay was 35, using data representing mostly the youngest part of the population, which exhibits the highest growth rates (individuals < 6 mm). These values are related to the reproductive period of the species and to regional biological and oceanographic features present in this bay. We consider these values of larval Pg as an index that in Bahia de La Paz the production of Nyctiphanes simplex is comparable to that of upwelling regions and that the bay functions as a nursery and feeding area for many other species, such as sardines, whales, manta rays, and zooplankton.

UNUSUAL LARVAL GROWTH PRODUCTION OF NYCTIPHANES SIMPLEX IN BAHIA DE LA PAZ, BAJA CALIFORNIA SUR, MÉXICO

Nyctiphanes simplex is one of the few neritic euphausiid species in the world. This species frequently dominates the nearshore waters of the Gulf of California, the transition zone of the California Current, and the eastern tropical Pacific off Ecuador and Perú. It has a high production in coastal and upwelling waters and plays an important role in the trophodynamics of these areas. This work examines the larval growth production (Pg) of Nyctiphanes simplex in Bahfa de La Paz, Baja California Sur, Mexico. The data were collected during 7 oceanographic expeditions from February-August 1990. Except for August, larval growth production remained at high levels in the bay. Daily and annual growth production for both transects were estimated. The mean annual larval Pg value for the entire area was 195.7 mg·dw·m-3 yr-1, the highest value recorded for the genus Nyctiphanes. The annual mean P:B ratio (defined as the ratio of annual production to average annual biomass for a particular species) for the bay was 35, using data representing mostly the youngest part of the population, which exhibits the highest growth rates (individuals < 6 mm). These values are related to the reproductive period of the species and to regional biological and oceanographic features present in this bay. We consider these values of larval Pg as an index that in Bahía de La Paz the production of Nyctiphanes simplex is comparable to that of upwelling regions and that the bay functions as a nursery and feeding area for many other species, such as sardines, whales, manta rays, and zooplankton.

Response of the Eastern Tropical Pacific to Meridional Migration of the ITCZ: The Generation of the Costa Rica Dome

Abstract A regional ocean circulation model with fine horizontal resolution has been developed in order to obtain a coherent seasonal picture of the eastern tropical Pacific off Central America. The Costa Rice Dome with a cyclonic circulation grows rapidly in late spring off the Gulf of Papagayo and matures in summer and early fall in accord with the northward migration of the ITCZ. In winter strong northern converging in the southernmost ITCZ from three passes in Central America excite warm anticyclones nonlinear eddies confined in the upper layer. Those anticyclones are identified as the intermediate geostrophic eddies by Matsuura and Yamagata. The Costa Rica Dome is eroded in winter and early spring by the westward propagating warm anticyclones but, at the same time, a new embryo of the dome begins to evolve from the westward propagating cold cyclonic eddy excited off the Gulf of Papagayo by the northers. The Costa Rica Dome can be maintained by the winds with the cyclonic wind stress curl in summer on...