Recruitment patterns of the pink shrimp Farfantepenaeus duorarum in the southern Gulf of Mexico

Fisheries Research 65 (2003) 81–88

Recruitment patterns of the pink shrimp Farfantepenaeus duorarum in the southern Gulf of Mexico Mauricio Ram´ırez-Rodr´ıguez∗ , Francisco Arregu´ın-Sánchez, Daniel Lluch-Belda Centro Interdisciplinario de Ciencias Marinas del Instituto Politécnico Nacional, CICIMAR, Mexico P.O. Box 592, La Paz, 23000 Baja California Sur, Mexico

Abstract Analysis of monthly recruitment of pink shrimp Farfantepenaeus duorarum (Burkenroad, 1939), to the Campeche Sound in the southern Gulf of Mexico from 1969 to 1994 shows high recruitment from July to November and lower recruitment from December to June. Recruitment has declined since the late 1970s, plummeting since the mid-1980s and reaching particularly low values from 1991 to 1994. From 1970 to 1985, the recruitment pattern showed lower than average values from October to February and higher than average values during April to September. From 1986 to 1994, the opposite pattern was the norm. The results suggest that long-term environmental change is related to seasonal changes in the recruitment pattern of juveniles and to a long-term decline in recruitment and catch. © 2003 Elsevier B.V. All rights reserved. Keywords: Recruitment patterns; Pink shrimp; Farfantepeaneus duorarum; Gulf of Mexico

1. Introduction Average annual landings of pink shrimp, Farfantepenaeus duorarum (Burkenroad, 1939), in the Campeche Sound, along the southern Gulf of Mexico, fluctuated around 20 000 t from 1965 to 1975 and were 90% of the total shrimp catch in the area. Since the late 1970s, landings have been declining and the rate of decline has accelerated after 1986. Since 1994, the Mexican government applied spatial and temporal closures but the resource has not shown any recovery (Ram´ırez-Rodr´ıguez, 2002). Less that 2000 t of pink shrimp were landed in 2001. The collapse of the fishery has been related to recruitment failure caused by either growth or recruitment overexploitation (Gracia, 1995; Arregu´ın-Sánchez et al., 1997a,b; Gracia and ∗ Corresponding author. Tel.: +52-612-1225344; fax: +52-612-1225322. E-mail address: [email protected] (M. Ram´ırez-Rodr´ıguez).

Vazquez-Bader, 1999; Castro-Meléndez et al., 2000; Ram´ırez-Rodr´ıguez et al., 2000). The pink shrimp fishery of the Tortugas Bank, in southern Florida, also experienced a decline trend that was associated with the effect of environmental forcing on recruitment (Browder et al., 1999; Ehrhardt and Legault, 1999; Fourqurean and Robblee, 1999). In the Campeche Sound, pink shrimp spawn all year, with peaks during summer and fall (Gracia and Soto, 1990). Migration towards nursery areas is related to the rainy season, primary productivity in the southeastern Gulf of Mexico and ocean circulation patterns. Peak juvenile recruitment periods occur during spring–summer and fall–winter and have been related to rain, river runoff and northerly winds (Arregu´ın-Sánchez et al., 1997a; Gracia et al., 1997). Ram´ırez-Rodr´ıguez and Arregu´ın-Sánchez (2003b) found that the spawning stock–recruitment relationships of pink shrimp in the Campeche Sound fit the Beverton and Holt model reasonably well,

0165-7836/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2003.09.008

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which indicates that recruitment is particularly affected by adult stock size and the environmental conditions experienced by each cohort during development. In the present study, we explore the relationships between long-term environmental trends and the seasonal recruitment patterns of juveniles to the fishing grounds in the Campeche Sound that could be related to the decline of the fishery in the area. 2. Materials and methods Monthly recruitment of 3-month-old individuals to the Campeche Sound was estimated for 1969–1994 through age-structured virtual population analysis, VPA (Ram´ırez-Rodr´ıguez, 2002; Ram´ırez-Rodr´ıguez and Arregu´ın-Sánchez, 2003b). Basic data was catch by commercial category (number of tails per pound) recorded at the Campeche and Ciudad del Carmen ports. Shrimp were aged following Castro and Arregu´ın-Sánchez (1991) using the von Bertalanffy growth equation (Ram´ırez-Rodr´ıguez and Arregu´ın-Sánchez, 2000), and a catch-at-age table was created from the landing-by-size data (Nance et al., 1989). This table was used to generate a catch-by-age matrix, assuming a stable growth pattern. VPA was solved following Hilborn and Walters (1992), using a maximum age of 11 months and natural mortality values given by Ram´ırez-Rodr´ıguez and Arregu´ın-Sánchez (2003a). To calibrate the VPA, the calculated catch-at-age structure from the Baranov equation was adjusted to the observed data, solving the objective function through the Newton algorithm (Lassen and Medley, 2001):
 2 Cobs − Cesp = MIN {Nt , Ft } (1) This procedure has implicit in it that catch is an index of stock abundance and was used because the lack of fishing-effort data and indices of relative abundance impede adoption of an ADAPT-type procedure (Lassen and Medley, 2001). As pink shrimp fishing mortality has been high (Castro-Meléndez et al., 2000; Ram´ırez-Rodr´ıguez, 2002), the method is a fairly robust procedure for examining historical trends in abundance and fishing mortality (Megrey, 1989; Quinn and Deriso, 1999).

Since pink shrimp recruitment occurs every month, we estimated the monthly proportion of recruits (recruitment pattern) in a biological year from October to September (Ram´ırez-Rodr´ıguez and Arregu´ın-Sánchez, 2000). To determine whether the recruitment pattern changes between years, we used a K-means test based on the matrix of percentages of the annual recruitment per month and season (StatSoft, 1999). To explore the likely relationship between recruitment and environmental variables, we used monthly means of sea surface temperature (SST) and salinity at Ciudad del Carmen, on the Campeche Sound coast, from 1969 to 1991 (kindly provided by the Mexican National Tidal Service, “Servicio Mareográfico Nacional”). The anomalies for each series were estimated as departures of the monthly averages, and the high-frequency interannual variation filtered with a 120 term Hamming filter (Blackman and Tukey, 1958), equivalent to 10 years. We examined the simple correlation coefficients between the full array of series and searched for autocorrelations and cross-correlations between the series. As multiple regression analysis permits evaluation of the partial correlations, and also their relative weight, we fitted recruitment as a function of both salinity and temperature for the different series; raw data, anomalies, filtered anomalies (low frequency interannual variation), and residuals (high-frequency interannual variation).

3. Results VPA showed recruitment during every month of the analyzed period. From 1969 to 1977, the average monthly recruitment was approximately 71 million individuals with a predominant peak in September–November, except in 1978 when the maximum was observed in April and in 1979 when a peak occurred in February and another in September. Afterwards, a decline in recruitment extended until April 1980, followed by an intense recovery until October of the same year. Recruitment was low during 1981 and 1982, and showed an increasing trend from November 1982 until November 1985 when its magnitude was about the same as those of the highest months at the beginning of the time series. However,

M. Ram´ırez-Rodr´ıguez et al. / Fisheries Research 65 (2003) 81–88

83

140 10

9

11

11

10

11

4

2

7901

Recruits (millions)

11

7801

10

120

11

11

9

10

8

10

11

9

10

10

100 80 60 40 20

9401

9301

9201

9101

9001

8901

8801

8701

8601

8501

8401

8301

8201

8101

8001

7701

7601

7501

7401

7301

7201

7101

7001

6901

0

Year-Month

Fig. 1. Pink shrimp F. duorarum monthly recruitment to the Campeche Sound, and 12-order moving average, from January 1969 to December 1994. Numbers above the curve correspond to the month of maximum recruitment per year.

recruitment plummeted afterward during the 1990s to its lowest values. The decreasing trend was stronger from 1986 to 1991; thereafter recruitment remained low and apparently stable, until 1994, the last year of the time series (Fig. 1). The annual cycles of salinity, temperature, recruitment and the filtered series are shown in Fig. 2. The residuals of the smoothed series were equivalent to the high-frequency interannual variation. Simple correlation coefficients between the full array of series demonstrated that only salinity and temperature

were correlated in the raw series (Table 1). Recruitment anomalies were directly correlated to salinity and inversely to temperature anomalies. Filtering for high frequency results in high correlation of all the series, temperature was inversely correlated to both salinity and recruitment, whereas recruitment and salinity were directly correlated. The high frequency series were either not or very poorly correlated. In general, there was quite a strong and inverse relationship between temperature and recruitment (r 2 = 0.94).

Table 1 Simple correlation coefficients between series of salinity, temperature and recruitment of pink shrimp in the Campeche Sound sal sal tem rec asal atem arec asalhs atemhs arechs asalhf atemhf arechf

tem

rec

asal

atem

arec

asalhs

atemhs

arechs

asalhf

atemhf

arechf

0.28

0.03 −0.11

0.63 −0.06 0.37

0.03 0.81 −0.30 −0.09

0.28 −0.25 0.88 0.41 −0.37

0.31 −0.25 0.78 0.49 −0.35 0.88

−0.30 0.27 −0.78 −0.48 0.36 −0.90 −0.94

0.27 −0.24 0.79 0.42 −0.31 0.91 0.91 −0.97

0.58 0.03 0.08 0.93 0.05 0.09 0.13 −0.13 0.09

0.12 0.78 −0.09 0.04 0.96 −0.13 −0.09 0.08 −0.04 0.09

0.13 −0.11 0.51 0.14 −0.26 0.56 0.26 −0.20 0.16 0.05 −0.22

Statistically significant values are shown in bold. sal: salinity; tem: temperature; rec: recruitment; asal: salinity anomalies; atem: temperature anomalies; arec: recruitment anomalies; asalhs: filtered salinity anomalies; atemhs: filtered temperature anomalies; arechs: filtered recruitment anomalies; asalhf: high frequency salinity anomalies; atemhf: high frequency temperature anomalies; arechf: high frequency recruitment anomalies.

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Fig. 2. (A) Seasonal cycles (mean monthly values) of pink shrimp recruitment, SST, and salinity patterns in Campeche Sound. (B) High-frequency filtered anomalies of recruitment, SST, and salinity.

The autocorrelation functions resolved the annual cycle (lag = 12) for the raw series as the dominant feature and also for temperature anomalies, in spite of the elimination of the annual cycle. For salinity and recruitment anomalies, no clear autocorrelations were found. The autocorrelations of the filtered series showed no lags. Cross-correlations showed that raw series were dominated by the annual cycle (lag = 12), with no consistent lags for the anomalies series. In the multiple regression analysis, recruitment was significantly correlated to salinity and temperature when the annual cycle was eliminated (r 2 = 0.27), and increased when the high-frequency interannual

was filtered (r2 = 0.88). This analysis confirmed that temperature (partial t = −34.96) is far a better predictor than salinity (partial t = −2.18). The average seasonal recruitment pattern for the full series showed high recruitment from July to November, except during 1978, and lower recruitment from December to June. Results from K-means group analysis sustained the hypothesis that the monthly proportion of recruits could change over the biological years yielding two recruitment patterns; Type I shows lower than average recruitment proportion values from October to February and higher than average values from April to September; Type II is the opposite (Fig. 3). Type I was the norm from 1970 to

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Recruitment proportion

0.16 0.14 0.12 0.10 0.08 0.06 0.04 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Fig. 3. Types of recruitment patterns of pink shrimp F. duorarum to the Campeche Sound. Type I (bold line), Type II (thin line), mean (dotted line).

85

that is, at the time when the recruiting cohort was born (r = 0.82, P < 0.05). The lowest recruitment levels of this group were associated with SST values of 28.5–29 ◦ C. Mean recruitment originating during the fall–winter period ranged from 13 to 72 million individuals (mean = 48 million, s = 17) and showed no significant relation to SST (Fig. 5A). July–November recruitment was significantly correlated to salinity 3 months before (r = 0.58, P < 0.05), during which salinity ranged from 30 to 39 (mean = 34, s = 2.7). During December–June, salinity ranged between 22 and 32 (mean = 27, s = 3) and there was no significant correlation with recruitment (Fig. 5B).

4. Discussion 1985, except during 1978 and 1981, whereas Type II dominated after 1986, except for 1993. Recruitment was defined as 3-month-old individuals, who were, thus, born during spring–summer and fall–winter. Both groups show a declining trend in recruitment, but the first group increased during 1981–1985, whereas that of December–June remained approximately stable. From 1986 onwards, recruitment in both groups declined more rapidly (Fig. 4). Mean recruitment resulting from the spring–summer cohort ranged from 24 to 100 million individuals (mean = 67.6 million, s = 23.5), and was significantly correlated to average SST 3 months before,

The average recruitment pattern of pink shrimp in the Campeche Sound agrees with that described by Gracia et al. (1997). From July to November, as salinity tends to decrease and SST is warm and stable, recruitment increases. This recruitment is significantly correlated to SST and salinity from April to August (spring–summer). Recruitment shows no clear trend during December–June, whereas SST and salinity increase from their lowest values during this period. Recruitment levels during this period are lower than those during July–November. No significant relationship existed between the environmental

120 100

Recruitment

80

60 40 20 0 69 70 71 72

73 74 75

76 77 78 79

80 81 82 83

84 85 86 87

88 89 90

91 92 93 94

Year

Fig. 4. Pink shrimp recruitment per month clusters from December to June (thin line), from July to November (bold line).

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120 y = -10.127x + 319.49 R = 0.27

Recruits (millions)

100 80

y = -36.549x + 1080.5 R = 0.82

60 40 20 0 25.0

25.5

26.0

26.5

27.0

27.5

28.0

28.5

29.0

29.5

SST -3 months

(A)

y = 5.3134x - 117.89 R = 0.58

120

Recruits (millions)

100 y = 2.7501x - 30.64 R = 0.46

80 60 40 20 0 20

22

24

26

28

(B)

30

32

34

36

38

40

Salinity

Fig. 5. Correlation between pink shrimp mean recruitment to the Campeche Sound. (A) During July–November (circles), December–June (rhombus), and SST 3 months previous. (B) During July–November (circles), December–June (rhombus), and salinity 3 months previous.

indices during fall–winter, suggesting the occurrence of more variable and less favorable conditions. Pink shrimp recruitment may be a seasonal function of parental stock. However, environmental effects on recruitment should be expected to be different for each shrimp life stage and could be related to habitat suitability as well as to variables related to the movement of the eggs, larvae, and juveniles, decadal changes in larval retention mechanisms over wide areas of the shelf, hurricanes (both frequency and intensity) and their impact on coastal circulation pat-

terns during spawning, and the volume of fresh water run-off (Costello and Allen, 1970; Bielsa et al., 1983; Garcia and Le-Reste, 1981; Garcia, 1984; Criales and Lee, 1995; Browder et al., 1999; Ehrhardt and Legault, 1999). Our results show that, in the long term, there has been a major change in the environment, mostly indicated by a sustained temperature increase, and that recruitment has changed consistently whatever the original cause may be. Our findings also indicate that changes in the recruitment pattern occur mostly as a

M. Ram´ırez-Rodr´ıguez et al. / Fisheries Research 65 (2003) 81–88

result of changes in the spring–summer cohorts. During 1970–1985, when the declining trend of recruitment was moderate, the Type I pattern dominated. Conversely, the Type II pattern was the norm from 1986 to 1994. The Tortugas pink shrimp stock decline from 1960 to 1993 was associated with a decrease in the most abundant cohort, which recruited during July–December. The cohort occurring from January to June remained relatively stable during the 1980s and 1990s (Sheridan, 1996; Ehrhardt and Legault, 1999). A decline in recruitment during the 1980s and 1990s was related to ecosystem deterioration in the Florida Bay, the main nursery area (Browder et al., 1999; Fourqurean and Robblee, 1999). Although oceanographic and climatic conditions in Tortugas and Campeche are different (Wieseman and Sturges, 1999), the pink shrimp stocks declined simultaneously in both areas. This suggests the existence of a long-range phenomenon that differentially impacted survival and reproduction of the cohorts, particularly those originating during spring–summer. Though the Florida stock has shown signs of recovery since the mid-1990s, the Campeche stock has remained at low levels, a phenomenon that Ram´ırez-Rodr´ıguez and Arregu´ın-Sánchez (2003b) believe is related to recruitment overfishing and continuing unfavorable environmental conditions.

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